2 years?! Whoa, that is a lot of time, I remember browsing /3dcg/ and then seeing Rose. From then I was hooked. Congratulations and thank you for the awesome renders you've made.

Thinking about a torture that I don't know if it's possible, however I will say it.

So, the girls are in a camp in the forest. There must be some source of water, maybe a shallow lake. Well, make some of the technicians or torturers install a chain link on the lakebed. Then announce a "competition".
The competition is to avoid death by drowning. The girls share a rope that passes through the underwater chain link. That same rope is tied around their necks. Also, the girls are kneeling on the lakebed, tied in place. Their hands and feet are tied too, so the must use only their torsos to avoid drowning. If one girl raises, the other one lowers. The water must be at their chin level when fully raised, so only one girl can "breathe" (remember, the rope will strangle her) without too much trouble. The winner is the one who doesn't drown. However, the winner will be asphyxiated by ramming a massive dildo in her throat untill she passes out and dies.