If i get it right what you want is to make 2 apps run in fullscreen mode in 1 monitor. It is possible in some Linux versions as Ubuntu with multidesktops. Well both the apps are running in 1 screen, but not at the same time. Multi desktop can be done in Xp/vista with some software, like this free one:
MultiDesktop Manager 0.3.8 (Right click-save target as...)
Or you can use 'dual screen' mode.
If so theres what you can do (for xp. In vista its similar):
-Plug-in the 2nd screen.
-Go to your desktop and click settings.
-Click the 'settings' tab.
-Click in the blue square with a '2' on it.
-Check the 'Extend my desktop onto this monitor'.
-Apply.
-To make 2 apps run in full screen just drag 1 to the 2nd screen, change it to fullscreen then turn the other app to fullscreen too.
I hope this helps.