If you are runnin 2 285s in sli both monitors will need to be run off of the top or 1st video card. Make sure you are running the latest drivers from nvidia.
After hooking up the second and booting go into nvidia control panel to the multiple monitor setting. The rest is easy.
More information on this subject can be found herehttp://www.slizone.com/object/sli_multi_monitor.html#confoptions