Dual monitor doesnt work with Additional Driver [trying again]

[oxy]

Hi

***** reviving the thread because was unsolved ******

I have a perfectly working dual-mon syst but when I
install proprietary drivers it stops working.

============ details ===================
Goto gnome-control-center > Displays > unclick Mirror monitors and get:

     " The selected configuration for displays could not be applied!
       required virtual size does not fit available size:
requested=(2560, 1024),
       minimum=(320, 200), maximum=(1280, 1280) "

I press [OK] and get:

     " Failed to apply configuration: %s
       Bus.Error:org.gtk.GDBus.UnmappedGError.
       Quark._gnome_2drr_2derror_2dquark.Code3:
       required virtual size does not fit available size:
       requested=(2560, 1024), minimum=(320, 200), maximum=(1280, 1280) "

Deactivate Add driver, reboot and unmirrored dual mon works again.
Any help? Thx...

System: Ubu 12.04 64Bit
Monitors: IIyama prolite E19065 (2x)
Videocard: Radeon HD 6870