Depends on the game. Some games will allow you to use dual monitors, while others cant support that high of a resolution, so it can only be displayed on one monitor.
i wouldn't suggest dual monitor for gaming unless there is less that a mm between the pixel of the two screen because the crosshair would be in the gap otherwise
I second staying away from dual monitor gaming. I have tried it in the past, and many problems do arise, unless it is done properly as heyman says, with 3 monitors. That way the crosshair would not be cut off.