> when reading this story I wonder what if they manage to lose their body parts what happens then?

Normally? They'd grow back, Alyssa's recent "situation" notwithstanding. Sure, hollow yourself out *enough* and there'll be no source of energy and mass to do the repairs, but if they lose a limb it'll just regrow.

If they somehow do themselves enough damage to be non-viable? Brain contents is backed up to DNA, which can be used to regrow a body at any medical facility with only a moderate amount of parental badgering. The one time Alyssa and Alex managed to simultaneously kill each other, it took their parents two weeks before they got around to resurrecting them.

It was the middle of summer vacation, see.

Fall into an incinerator, or active volcano? Then it gets a little more philosophical. They can still be resurrected, but using backup data which may be weeks or months old in the worst case. Alex managed that *once*, which is why he is now unambiguously the younger twin.