Hmmm, I use two screens, and I've always had a fullscreen game on one screen
Would be kinda pissed off if my card wouldn't allow me to though,
playing an FPS with a big black bar in the middle of the fucking crosshair
Anyway, I'm not familiar with nVidia cards, but there should be an option to put it in dual-mode.
Maybe you should set it in single mode, and see if that helps.
Maybe that way Windows will recognize two monitors, like mine does, and let you choose to "Extend the windows desktop this monitor"...
Let us know if you succeeded