@phantori: in a way this reminds me to a mix between Ranma and Nikki/Jessica from Heroe's XD
my only question is, how did the scientist mix the 2 siblings? 
I want to keep it with at least some logic in it so even if far fetched or high class sci-fi I want to know the theory behind it XD

with Bone its pretty clear, genetic manipulation of the bone structure, increasing its density and mass, even the hardened skin can be connected that bone cells are infused with the skin cells.
in normal science this would be impossible but I can see the theory behind it XD
I myself cant find it with the sibling fusing so I wondered what you thought about it XD

I can see how they switch, with the body change included, hormonal balance (also explains how the brother can be so violent on top of it)
oh and just asking, these blasts of excess energy are they close range or long range?

beyond all that both are good, so its just simple asking on my part (curiosity and as said, some logic XD)

last but not least, welcome Whirlwind ^^
I hope so as well that this will be active for a long time ^^