Good luck. It's bad enough to get X-Windows working correctly on 1 monitor. (in 4.x anyway). I don't know how ATI is, but with NVIDIA you can install the drivers, (init 3) switch the driver name in /etc/X11/xorg.conf (I think) then enable it with the driver. Not totally sure about ATI. If their BSD/Unix support is as terrible as Linux, I doubt you'll be able to do it.