since you are only going to be playing two characters you don't even need that great of a PC or video card.
I would use one monitor because it's a pain to move your mouse to the alternate screen.... I doubt you will do that too much but still. Depending on the software you use it may be easier to use one monitor where you can instantly click on the other character and make it the largest part of the screen. ISBoxer and keyclone let you do that where there is a main sized part of the screen and smaller alternates with switching between them, I'm sure some of the other software does too.