If the game doesn't offer the twin resolution, the easiest way is to run that game in a window and just drag the edges your other monitor. However, some games take a massive fps hit when playing in window...
I believe that games typically ask for the 1st graphics card (unless specified) and the 1st screen. The driver makers need to tweak the drivers in order to support this functionality (what they're doing with Eyefinity and whatever nvidia's is called).