SOME games will only play on one. Many don't support it, but still there's some out there that support MM's.
The only FPS I've tried this on was TeamFortress. It has field of view (FOV) commands that opened up the sides a bit, but the reticle and the enemy you're trying to frag are still in a bad area. I've caught myself trying to look behind the band of plastic "hiding" the enemy during play :p
At the time, I knew of a way to rotate the FOV off to the side a bit so the reticle was on one screen more than the other, but it was still very awkward and dual monitor FPS'ing didn't last past that day.
I don't miss the other monitor outside of Flight Sims and when I'm chatting in IRC or whatever.