From what I have tested using 2 monitors and playing both on 1 PC is that the 'main' monitor will have nice FPS while the secondary will suffer and the fps gets cut in half or maybe even more. So it will appear somewhat choppy.

As well melee classes aren't that effecient for dual-boxing or higher if you play more than 1.

First, try out running 2 wow clients at the same time and see if you can bear the performance, from there I would suggest testing different classes to see what you like. If you can manage to get 2 rogues to always stay on top of a target it may be possible