Well, from what I understood, the mac pro (intel) can only run a card that has flashed bios to run on a mac, or a card direct from apple.com. But even in the flashed bios card community I can't find anything on multiple cards.
Really... that is news to me. But, you'd know better than I would.
I was just googling, and it does look like info on Mac graphics cards is slim...
Sorry, I guess I can't be that much help on this one.