Why? What is the point of doing this? I'm really curious because I just can't work out the whole thing at all. Your ranged DPS parse similar and your melee parse similar and you have one of each in both pairs - so what's the point in switching them around?
I'm sure I'm missing something here as I'm not the sharpest tool in the shed but can't figure out what is it.
Because they're taking two double destructions. In Nightmare mode, the debuff from double destruction does not wear off before the next double destruction is cast.
If you have the debuff and take DD, you get one shot. So they switch sides after every double destruction so they always have people without debuffs taking it.