Which driver are you talking about?
I use the drivers from AMD, proprietary binaries.
There are a number of ways involving aticonfig/xrandr. Apologies if I'm a bit hasy but some rubbish gear from Gigabyte took some time to sort out.
If you run aticonfig --initial=dual-head --something=horizontal then start Cm you will end up with two different screens, one one each mnitor, unable to move a window from one screen to the other. The advantage pf this is you can swap to a different workplace on one screen while the other stays the same.
If you start X then run aticonfog/xranad after X starts, then you'll end up with one screen ov
xfce handles the second ocat and the first basically(workplace laves van not be changed).