I've never tried this but can't you run two cards in an SLI motherboard as non-SLI? That way you should have 4 independent displays.
Bingo,when i had vista installed for example, I couldnt get the right NVidia drivers for SLI, hence, the grafic's options showed me 4 monitors
But, adding a pci grafic's card should work already, but to be sure, I'd say, if your card thats already in the pc is Nvidia, make the pci one be an Ati card vice versa. (just to be sure the grafic's driver doesnt completely mess up)
The other way around this would be to get a pci card with the same gpu.
I'm running a pci-e x1300, and a $80 pci x1300, and they play nice together.