if you have two monitors you can play across both if "span" your desktop. by default, when you plug in your second monitor, windows just creates an entirely separate, secondary desktop with nothing on it. no clock or task bar.
lets say your monitors are 1080p. by default windows will create 2 desktops, both would be 1920x1080 in resolution and when you game, you will only be gaming on the monitor/desktop designated as the PRIMARY.
if you span the desktop, you will get a resolution of 3840x1080, your game will only see it as one monitor and will span across both monitors.
but playing with 2 monitors will leave the bezels going right down the middle of your vision and it will be a pain in the rear playing any sort of game. even...