It is currently impossible to install two video cards at the same time in a Mac and have them work together on the same screen. It would require a compatible chipset,which could be expected in the Intel followers of our G5 Powermacs.
Yet if Apple wanted it, a solution could be found.

This card is a GeForce 7950 GX2. As others did before, it holds two 7900 CPUs working along.
What's new with it is that it directly interconnects the two CPUs, while the computer in which it is will only see one.
No more specific drivers or SLI/Crossfire required to use it.
Any PCI Express machine could take advantage of it.
We now know that Apple won't listen to their clients suggestions (as a little girl recently learnt at her expense). But having such a card would delight us.