So let me clear it for you. I can clear it using maths as well but for the starters let me try it using words only.

Double of half is full. So if I'm getting beaten half to death, my twin would get beaten full to death!

I hope you understand now. If you still don't, let me use maths.

Consider health of both twins(T1 &T2) to be exact same at 100.

Let us assume that it takes 50 punches to get T1's health down to 50. (Considering 100 as full health and 0 as death, 50 has to be half to death)

Now, since T2 get twice of T1, T2 would recieve 100 punches which will bring his health to 0 and hence, death!