Not many games supports 2 monitors. 3 monitors, yes, but not two.
Playing a 1st person shooter game spanning two monitors will be terrible. The targeting reticle will be split between the two screen; half on one half on the other. It would also be separated by the monitors' bezels which combined can be around 1" thick. You would think that a game like Civilization 5 would support dual monitors... Nope, 1 or 3 monitors.
I have a dual monitor setup, but I only use the 2nd monitor when I am doing a lot of multi-tasking like having multiple browers open for doing research, or if I wanna watch a movie while doing other things on my PC.