The two monitors 'share' the vid cards ability to output frame rates though, right?
Meaning, doesn't having to drive both sets of images reduce the frames per second in your game ?
I would definatly assume your vid card/processor still has to use resources to generate images on both monitors. However, unless you're running two video intensive games/programs on each monitor, I don't think it would be an issue.
I mean the CPU/RAM/video RAM needed to generate the desktop or a website on your secondary monitor has to be so minute you probably wouldn't notice any difference.
Hell, I've played World of WarCraft and WarCraft III at the same time with no problems, and my machine is dated at best.