Dual Monitor setup with ATI Radeon doesn't work on Xubuntu 11.10. At least not on the 64bit edition.
The "Additional software" utility installs the proprietary driver, but it doesn't work.
In Settings, there are two menus created after install:
1. AMD Catalyst Control Center
2. AMD Catalyst Control Center (adminstrative)
The first one doesn't let you modfy anything, the second one doesn't even run. (read more)