I have two monitors and a ATI Radeon 5750 video adapter.
Using ATI's catalyst control center I am able to configure the displays so that the desktop spans over the two monitors.
The multi-desktop settings are preserved when logging with KDE. It was also working properly with gnome and unity (when i used to have them installed).
BUT: when logging in with XFCE the second monitor is cloning the first (instead of creating an extended desktop).
I can enter into ATI's control center and reconfigure the desktop. But the settings are lost when i logoff !
Does anyone have any clue about what is happening here ? Why does the Xfce desktop uses some wrong multi-monitor settings ?
This problem appeared first when i upgraded to 11.10 (about 2 months ago). And now after upgrading to 12.04 the problem persists.
Follow instructions here to update to Xfce 4.12., which will provide a new minimal display settings window, and options for dual monitor, which were not present in Xfce display manager in older versions.