Hmm. It would be pretty weird if it affected you but not me, especially if it was more than a marginal difference. My experience is that I went from playing when I didn't have dual monitors, to getting another monitor and not noticing the difference if there was one. I'm going to do go and test to see if I get an actual FPS difference or not. I'm bored... so why not?
... time lapses ...
With dual monitors:
Flying around shatt: 15-20fps (average around 15)
Flying around terokkar: 25-30fps (average around 30)
Without dual monitors:
Flying around shatt: 15-20fps (average around 15)
Flying around terokkar: 30-40fps (average around 40)
I'm pretty sure this backs up my thought of the limiting factor being the processor... even though it is late, shatt has a lot more going on, and stays about the same. However, out in the wild, I get about 10fps more on average. The already crappy city performance is probably why I didn't notice when I first added the second monitor.