OP here again. Back with another chapter, and a username. Sorry for the long delay.

---

Their faces mere centimeters apart, Alex and Alyssa were frozen as their brains attempted to process the myriad sensations that were suddenly bombarding them. They were gripping one another rather tightly. Their hearts were pounding. Their clothes were in disarray. Someone was gripping them by the hair. There was something else, too—an alien, but none too unpleasant feeling.

There was a voice. Alex slowly turned towards the source, and saw a man in a uniform. He was the one gripping their heads.

“--And far be it from me to tell you what you can or can't do in the privacy of your own homes, but I need to see tickets, and your bodies aren't the most... cooperative,” he continued, gingerly releasing his grip. “So you lovebirds better keep your heads on.”

Alex's mind was still reeling, but he picked up something about tickets. Right. He had their tickets. Left pocket. Any distractions to stave off the inevitable horror of confronting his situation were welcome, so he took his time searching his pockets. But the ticket wasn't in either pocket. He clearly remembered buying them, and that he didn't trust his sister to hold on to hers. Yet as he thought this, Alyssa procured both tickets from her left pocket and handed them to the man.

After giving the tickets a once-over, he bid them both farewell and made his exit, leaving the twins with little to talk about but their current sticky embrace.

Alyssa was the first to speak. “So... what happened?” she asked, despite having a pretty good idea. She had had a view of his head, and with it, his facial expression as he watched whatever their bodies had been doing. Were still doing. Fuck.

“Both of our necks were still loose, and you headbutted me,” said Alex in deadpan.

“I meant after that, genius.”

“We scuffled for a bit, and then it looked like we forgot what we were doing, and then we—uh, our bodies—just started touching each other--”

“No shit.”

“--and you undid my pants--”

“Bullshit! Like you could keep track of which one was which,” Alyssa mocked, noting their identical school uniforms.

“So you just admitted you're flat?” Alex replied, forgetting his shock for a moment.

“It's just these uniforms!” she said, flustered.

“Besides, mine is the one with a dick,” continued Alex, “and yours was the one that took it out and played with it--”

“Ugh, shut up!”

“By the way, how does your throat feel?” he said, goading her further. “It looked like a pretty tight fit.”

“I said shut up! You're just making stuff up!” Alyssa fumed. Despite herself, she couldn't help but reflexively clear her throat, causing her to turn even redder as she glared at her brother.

“Whatever! You sure must have wanted it, because look at us now!”

Alyssa shuddered. “About that,” she said, looking down. “Would you mind... pulling out some time today?”

As long as they didn't move too much, it had been possible to ignore the fact that they were joined together at the hips. They were seated facing each other along the length of the cushioned booth, his right and her left leg interlocking. His was on top, so it looked like the first move was up to him. He sighed, lifted his leg, and slowly backed away from his sister. Something felt... off. The only way he could really think to describe it was... inverted? It sure didn't feel unpleasant, though. Finally, the friction stopped, signaling that they had decoupled.

And Alex watched his rock hard cock spring out of his pants. And away from his body.

Instinctively, he reached for his errant member. Can't I use this thing once without it falling off?

But as he gripped and tugged on it, his sister let out a sharp moan. “What the hell was that? Why the hell is it hurting so much?” Lying on her back with an arm draped across her face, she couldn't see the source of her discomfort.

As memories of her past teasing resurfaced in his mind, Alex wasn't eager to tell her what had happened. Or at least, what he thought had happened, until he realized he was looking at the head of his cock. “Oh shit.” He plunged a hand down his pants. He knew what being dickless felt like, and that hole sure didn't feel this good (he had tried—all the more reason never to lose it, he concluded). As he looked down at his soaked fingers, he also spotted the sky blue panties he'd seen countless times—and was now wearing.

Just his luck. So the ticket enforcer couldn't tell them apart either. And there was no way to switch their heads without at some point having them both detached at once, which wasn't happening as long as their bodies had other priorities.

Looking back at his sister, Alex saw that she had finally found the courage to examine herself. As she stared at her new cock (and it stared back), the truth about their situation dawned on her as well. “Fuck. Fuck fuck fuck. Fuck!” she shouted. “I'm a girl, got it?” she growled, addressing nobody in particular. Then, lunging towards Alex, she grabbed hold of his chest, kneading the flesh so as to emphasize the existence of some soft tissue. “See these? Girls have these! How the fuck could someone screw that up?”

Alex winced. “Argh! dammit, you should know how much that hurts,” he said reproachfully, massaging his bruised breasts.

Alyssa promptly slapped away his hands. “No touching!” she commanded with a glare.

“Oh come on, Alyssa,” he said, rolling his eyes, “like I can avoid touching myself. What do you want me to do, anesthetize myself so I don't feel all your parts rubbing together?”

“Well when you put it like that, yes!” she said, burying her crimson face in her hands. “Ugh, how do you manage to make even that sound dirty?”

“You're one to talk,” he chuckled, “with your dong just hanging out like that.”

“AAAAAGH! NOT MINE! IT'S NOT MINE!” screamed Alyssa, attempting to cover the throbbing organ with her hands.

“Hey, calm down, I mean it. Do you really want to stay like that?”

“Well what am I supposed to do? It hurts like hell when I try to push it down!” she said, exasperated.

“Well duh. You've been rock hard this entire time,” he said. As she pinched the shaft experimentally, he gave a weary sigh. “Come on, don't tell me you didn't try anything last time we were like this.”

Alyssa narrowed her eyes. “Don't tell me you did,” she threatened.

“Look, do you want me to help you or not?” he asked, avoiding her glare.

“And how are you going to do that?”

Now it was Alex's turn to be embarrassed. “Well, it's... uh, I have to...” Alex glanced down again. Sure, it wasn't attached to him, but he couldn't help but feel some sense of ownership over his parts. That was his erection she was having. She, and by extension Alex, was painfully backed up, and he couldn't stand to see it. He knew just what to do to make himself feel better; the problem was that he quite literally wasn't himself. He'd just have to pretend he was. “How about you lie down first,” he suggested.

Alyssa did as instructed and lay on the seat, but without taking her narrowed eyes off him.

Alex doubted that he could convince her to look away and just trust him, so he tried to ignore her as he reached towards his sister. She gasped again as he gripped her by the engorged junk, but nodded for him to continue.

It's just like masturbation. I am not giving my sister a handjob, he thought without much confidence as he began pumping. This motion was one of the most familiar things in the world to him, yet there was no ignoring that it was an interpersonal act this time. No, it was more than that. However much he repeated to himself that this was his body and nobody else's, he wasn't identifying with it in this moment. He was a girl, giving a handjob to someone else. Someone she loved.

Alex shook his head violently. No. Bad body. I'm in control here. I'm just doing this because I have to, got it? He didn't know if it was listening, or if thinking thoughts at his body even made sense, but, for his part (and for his sister's sake), he needed to maintain his composure.

Alyssa moaned. “I don't even care how wrong this is. Just don't stop,” she pleaded, resting her arm across her forehead.

So with newfound determination, he continued jerking off his sister. Her breaths became shorter as his technique rapidly took effect—he knew how to make himself feel good, after all—and before long, he could see the telltale signs that his body was reaching its limit. His sister obviously noticed as well, arching her back and gasping. “Ahh... I'm gonna... I'm gonna--”

Alex realized he had forgotten about the mess. He quickly scanned their room for something to contain the explosion, but everything was still in their bags. By the time the first warning shot hit him in the face, he knew that he had only one choice.

He closed the distance between his face and his (currently her) dick in the nick of time, as load after bubbly load filled his mouth, throat, and airways. Spluttering and coughing, he attempted to swallow as much as he could as his sister emptied herself. Ever the selfless sibling, he took comfort in the fact that this semen was destined for his sister's stomach, which he didn't intend to keep for long.

Somehow, that didn't make this feel less degrading.

---

After a not-so-short bathroom break, Alex was somewhat refreshed. The taste was almost gone from his mouth, though he'd probably need a new toothbrush. More importantly, some routine self-dissection had revealed a womb filled with semen. He'd inverted it and soaked it in hot water, but he was still worried; human eggs don't exactly give up without a fight—so he'd heard. For now, it was crammed in his backpack between some ice packs. She probably wouldn't miss it for a while.

Also, despite himself, he'd stayed a bit longer to satisfy certain... urges. Despite all his teasing, Alex didn't think Alyssa was actually flat, and perhaps due to her malleable composition, clothing did have an unflattering effect on her bust. Looking down at himself in the nude, however... well, suffice it to say that he would have gotten hard if that were still possible. And he might have broken the no touching rule.

Hopefully, Alyssa would understand, after what he did for her.

But back in their first-class compartment, he saw that Alyssa was asleep under a blanket. They had another five hours at the leisurely pace this train was going. It's not like he had been expecting an uneventful trip, but this? As fascinating as it was to get another chance to explore this body, certain realities were beginning to set in. What would they do when they arrived? As if the first day at an elite boarding school wasn't daunting enough, now he'd have to survive a thorough physical examination and presumably explain to the administration why he was registered as a male.

They'd have to switch back again, but how? One careless move could set them back even further. This all happened because of a headbutt, for crying out loud.

Alex wracked his mind for an answer as his sister slept soundly.

---

Alyssa awoke to a clattering noise. She blearily opened her eyes, annoyed that her much-needed rest had been cut short.

“Alex?” she inquired groggily, still not sure where she was. Her brother was dumping the contents of a backpack on the floor, which was evidently what woke her up. As her eyes came into focus, she realized that he was naked. And a girl. And she was a boy. Nope, it wasn't just a bad dream. She sighed, but her breath caught as she noticed that she'd been undressed. How did he keep doing that without waking her up? “Mind telling me what's going on?”

Alex finally looked her way. “You want to be back to normal, right? Preferably before we arrive?”

“Duh. Every second attached to you is probably setting me back years,” she said, staring forlornly at the budding knockers currently adorning her brother.

Alex rolled his eyes, but went on. “And we can't just swap heads, unless you want to leave our bodies alone again.” Alyssa shook her head.

“That leaves two options. We get help from someone on the train, and entrust our bodies to some random stranger,” he said, rummaging in the pile of school supplies, “or we make our own help.”

Alyssa looked confused. “Can't we just do what we did last time?”

“You mean without the part where we tear each other to pieces and almost die? Well, that wasn't really a head swap, remember? That was something I could do by myself, so there was some extra... mixing and matching.” Alex seemed to finally find what he was looking for, and picked up what looked like a spool in one hand, and some plastic wrapping in the other. “Besides, I've got a perfect, simple solution.”

“My hero.”

“First, I've got to explain it to you, since I we won't be able to talk, when we're... uh, in the middle of it. And we need to coordinate,” he smirked. “All four of us.”

Alyssa raised an eyebrow. “Go on.”

“We don't want to relinquish control entirely, right? So we just need to do this incrementally.” He began unwinding the razor-sharp wire in his hands. “If we cut ourselves down the middle, we can just switch half of our heads on the corresponding halves of our bodies. The other halves can help, until it's their turn. Then we join back up. Any questions?”

“Yeah, a couple. First, why are you packing all that wire?”

“Don't you get bored on long trips?” Alex asked with a smirk.

She couldn't argue with that. “Okay, second question, why not just cut our heads in half and switch those in stages?”

“Then who has control of the body at the halfway point? For that matter, what happens if we accidentally merge brains?” They both shuddered.

Alex threw his sister the wire. “You go first. I need to be in one piece in case something goes wrong.”

Alyssa scoffed. “What, you think I can't cut myself straight?” She carefully lined up the wire between her (brother's) legs, not at all opposed to the idea of going first if meant proving how well she could mutilate her brother's body. Too bad she was the one who had to feel the wire sliding through her abdomen.

With care and attention, Alyssa guided the wire exactly through the middle of her brother's genitals, perfectly separating his testicles and cleaving the shaft in two down the center of his urethra. Alex cringed through the spectacle. “You could have just gone around that, you know. I've still got your body hostage, so play nice with mine.”

Alyssa stuck out her tongue as she continued to saw her way through his body. Soon the wire was in her territory, and she had to sit with her back against the wall to make sure her arms didn't end up askew, bringing the wire over and around the back of her head as if flossing in a particularly tricky corner. As the metal floss divided her lower jaw, drool began to spill down her neck, which she initially tried to stifle with her elbow before giving up. Finally, as the last fibers in the corpus callosum were severed, her eyes no longer moved in synchrony, and the two-handed sawing motion became a feat of coordination, until at last the last bit of connecting tissue above her forehead gave way. Half of her gave a triumphant smirk, while the other tossed the spool back to her brother.

Alex was mildly impressed. He had to admit, she made it look kind of easy. Just how much practice did she have? Another part of him was sure that if she could do it, so could he. And the plan would be a success. “Alright, now let's just wrap you up to make sure nothing falls out,” he said, picking up the plastic wrap.

His sister decided she should probably stand up for this. At least, half of her did. The other half watched as Right-Alyssa stumbled for a second on one leg before falling forwards, yanking out her left half's entrails while reaching for support before spilling her own all over the floor. On impact, her brain slid out of her skull, skidding into Alex's foot and leaving her lying face-down and twitching.

As a rather annoyed Left-Alyssa carefully pooled her share of Alex's guts back into her cavity, Alex quickly replaced Right-Alyssa's brain, and sealed both of their heads' openings with plastic wrap. Sealing his body was a lost cause for now. So much for easy, he thought.

“Can you still understand me?” He asked both Alyssas. Both nodded.

Satisfied, he prepared to split himself, making sure to mentally map out his exact movements from start to finish so that he didn't end up like Alyssa. Or rather, so that her body didn't end up like his. Ever the caring sibling...

The wire quickly settled into a certain groove, perfectly splitting his sister's vagina. “Fair's fair,” he said, addressing his glowering sibling. Or at least, the half of her that was glowering, and not poking at her brain. He soon regretted taking her genitals head on, however, as the feeling of cold metal sliding through his clitoris caused a brief moment of euphoria, accompanied by some obscene noises that, as a boy, he felt somewhat ashamed to make. Some undigested white goop trickled out of the gap once he passed through the stomach. He achieved the final stretch in much the same manner as Alyssa had, except that he made sure to remain seated while sliding two layers of plastic wrap between his halves. Too bad this was probably the first and last time he'd get to stimulate this many erogenous zones at once. Maybe if he got a girlfriend, he thought...

His ruminations were short-lived as Alyssa's impatient glare brought him back to his senses, and he pulled the plastic foil through the top of his head, pulling apart the excess wrapping and flattening it against the edges of his cut, from his crown to his labia. Aside from a faint glint of plastic, he looked like a perfectly intact girl (well, a girl with her brother's head). That is, until he wormed his fingers into the cut and pulled himself apart with a sound like tape being peeled, starting at the stomach and sliding his arms in opposite directions, exiting through his head and his sister's privates. Finally, he scooted apart, so that his sister's insides were on full display through the transparent membrane.

With a flurry of gestures, pointing, and finger counting, the two Alexes communicated a plan of action. First, Right-Alyssa and Right-Alex would work together to switch Left-Alex with Left-Alyssa. Then the completely male Left-Alex and completely female Left-Alyssa would switch Right-Alyssa with Right-Alex. Right-Alyssa made a visibly concerted effort to take the plan to heart, but finally nodded, instinctively holding her brain steady.

The half-siblings seated themselves on the floor in a small circle. With only one arm each, the they needed to grip each other by the hair. After a few tugs, the first round of heads came off, made easier by the fact that there was only half of the usual cross-sectional area to tear through. Right-Alex and -Alyssa carefully carried the heads to their destinations, making sure to do so before the headless bodies switched to autopilot. They waited until both left siblings had given a hand signal before letting go (Left-Alyssa flipped the bird, but that was good enough for him), then gave themselves up to Left-Alex. As Right-Alyssa felt her head tear off, she felt strangely comforted as she relinquished all control, letting her brother carry her back to the comfort of her body. Left-Alyssa dutifully tore off Right-Alex's head to make way, and after the last signal, the restored siblings located their partners, lined themselves up, and hugged themselves tightly. Alex couldn't help but feel proud, having executed his admittedly convoluted plan in under an hour. For her part, Alyssa was of two minds on the matter.

A few minutes later, and Alex could barely see the seam down his body. He gathered up the organs his sister had dropped (at least he knew they were all his—he'd been kind enough to avoid any spillage while handling hers, after all) and haphazardly threw them through an opening he made in his belly. He'd sort them out later, if necessary. More importantly, his shorn shaft demanded special attention; the poor organ had seen far too much action lately. Only after minutes of pinching and squeezing was he courageous enough to let it go.

His sister was fast asleep already, clearly still tired from her earlier interrupted nap. He sighed, got dressed, and snuggled under the blanket beside her. As often happened, the relief of having rebounded from yet another ordeal together made him feel strangely at peace. None of their fights up to now mattered. Their differences were inconsequential. They had a clean slate, and she felt warm and soft when he wrapped her in his arms. Soon, he, too, drifted off to sleep.

---

When Alex awoke, sunlight was streaming through the window. He checked his watch—7:30 in the morning. They were arriving soon, so he decided to wake up his sister.

“Hey Alyssa,” he said, shaking her gently. “You should get dressed. We're almost there.”

Alyssa turned towards him and opened her eyes a crack. She grunted defiantly and pulled the covers over her head. Alex sighed. Wasn't she supposed to be the one yanking him out of bed on school days? Whatever, he thought, if he could undress her without her consent, he could dress her without her consent. He pulled away the covers and instantly realized something was wrong.

“Aaaalex,” she whined in an uncharacteristically childish tone, keeping her eyes shut. “Five more minutes!”

Alex noted that she looked incredibly small. Her left arm and leg were missing, which was a mystery by itself. Did her limbs fall off in her sleep? But that wasn't why she looked small. She was definitely shorter, and thinner around the chest and hips. Like a ten-year-old. Convinced he was seeing things, he straightened her limbs (ignoring her protests) and and attempted to put her clothes on. Right away, he saw noticed that her panties hung off of her like a loincloth. This was definitely real.

“Sis, did anything... weird happen last night?” he asked hopelessly.

Alyssa grumbled, shaking her head. “Hrmph. Left arm fell off in my sleep I guess,” she speculated, gradually waking up.

Suddenly, a glimmer caught his eye. It seemed like her whole left side was covered in something shiny. He peeled some off around her belly, causing her to giggle violently.

It was plastic foil. His mind raced. Had she forgetten to remove it? But then she'd still be in two halves. And the foil was outside her body, covering her missing limbs. So her arm and leg couldn't have fallen off in the night...

At that moment, he heard a groan from right behind him.

“Alex? Are you up? What time is it?” A pause. “Hey, have you seen my right arm?”