Multiple Graphic Cards in Mac Pro

I know someone posted about using multiple Graphic/Video Cards in a Mac Pro. I wanted to know if you had one compatible one and one that was not. for instance if you were running a monitor off of one and had an additional Card that was only for Acceleration of C17, Would C1 know which card to use and ignore the other? I guess I am hoping someone from Phase One or perhaps someone who had done this would know.
Thanks
David