first of all, you need to be able to pay alot of attantion of both screens in the same time, thats kinda hard, but possible

2nd, you gotta use 2 mouses with spells binded to them. That will be far from enough to use even half of character's possibilies.

3rd. There is no way you can learn to use mouse with left hand anytime soon.

4th, I dont see how is it possible to move with both chars in the same time, target something different in the same time, cast different spells, etc etc etc

you will constantly fail to do right spell, move to right side etc.

I am pretty sure the idea is pretty weird and impossible for human. If you are naga-ogre with 2 heads and 4 hands - you can try.