Two monitors and a single X-Screen: How to change "full screen" center and resolution

view full story

http://unix.stackexchange.com – I have two monitors, both 1600 x 900, running as a single x-screen. When I try to run a full screen application (a game for example), the application full screens as though my two monitors were one monitor at 3200 x 900. The center is then of course the crack between my two real monitors. How can I run such applications so that they only use one monitor, 1600x900, centered on one of the real monitors? (HowTos)