I am not sure that the graphics card can support dual screen mode in anything but mirror mode. There should be an option in your display settings to change from mirror mode to extended desktop mode.
"Fix it 'til it Breaks."

Jacob - EiC & Director of Technology
Mac Pro Quad 2.66 - 4GB RAM
160 GB SATA RAID 1 - 650 GB Storage
Quad 19" Widescreen LCDs
Accessorized to the Hilt