I personally recommend multiple computers. You can get a second WoW box for about $200-$300 so it's not that spendy and it makes multiboxing a lot easier in my opinion (especially with keyboard broadcasting software).

If you do go with two games on one box turn off sound. For some reason having the sound on (even if it is disabled when in the background) kills your FPS on a single machine. I used to dual-box on one machine with sound on and I would get 10fps. Once I turned sound off I can 3-box on one machine and get 20fps on each.