One good computer is pretty ideal for it instead of multiple, unless you get into the realm of 5-boxing. I recommend using Autohotkey so you don't have to alt tab to the other screen for most functions. It's free, and requires a limited amount of programming knowledge to use. There is a program you can pay for called KeyClone that is pretty popular if you don't want to fool around with AutoHotkey. More info on this kind of stuff can be found at
www.dual-boxing.com.
Early on, I used to have a lot of trouble with autofollow. I figured out that having Vertical Sync on helps a lot with this, and as far as I understand it limits the FPS number which is why autofollow breaks less easily for your second toon.
Personally, I use one instance of WoW in full screen mode, and my second instance is in windowed mode(make sure it's maximized and not minimized or you'll experience slowdown for the second character). Having two windows in windowed mode seems to cause framerate issues for me to a degree.
Hope this helps... 2boxing can be quite enjoyable with the right setup, and freshens the game up a bit.