Nearly all remotely recent GPUs can drive three or more displays, no need to keep two GPUs in your system unless you have special reasons to do so, such as using the older one for PhysiX as Krypt suggested.
I'm running triple displays on my ancient HD5770. Even Intel's newer IGPs can do triple displays.
You should be able to run two GPUs in the same system but that would only waste power. Driving two displays from the same GPU has negligible effect on performance when playing a game on one display at a time. Also, most games will only run on the primary display adapter so, even if you move a game's window to your second display on the second GPU, the primary GPU will still be the one rendering it.