Make computer use 2 monitors as 1

Solution 1:

From your comments above it sounds like your computer has an older Radeon HD 4800. Starting with the HD 5000 series, ATI introduced a technology called EyeFinity, which allows you to use multiple displays as one large desktop. In this mode, applications see you as only having one giant monitor, rather than two separate monitors. Maximinizing a window will make it take up all of both screens.

In order for this to work well, you need monitors with identical vertical resolutions. Otherwise, EyeFinity will use the lowest resolution of the two monitors, and will stretch on the other one. Of course, this doesn't look very good.

Any ATI Radeon HD 5000 series or newer will support EyeFinity, but only certain models support more than two simultaneous monitors. If you think you'll be using 3 monitors in the future, make sure to get a card with at least one DisplayPort, as the third (and additional) monitors must be connected by DisplayPort.