It depends on your graphics card (or motherboard if you don't have a graphics card).
Do you have places to plug in 2 HDMI cables?
If so just plug both monitors in!
If you have a single HDMI and a single DVI, simply use HDMI to one monitor, and DVI to the other.
Concerning gaming on only 1 display:
If you only have 2 monitors, this will be the default setting.
Even if you have 3 monitors, you would need to setup Surround gaming (for Nvidia) and EyeInfinity (for AMD) to game on more than a single display.