Two of the original video cards in 2009 Mac Pro?
I'm assuming you have a Mac Pro 4.1 or 5.1 which uses a Nvidia GT 120 which on its own does not support SLI. When you referenced the technology that can be used to utilize multiple graphics cards, that is SLI for Nvidia and Crossfire for AMD.

Since you have an Nvidia card, you would use SLI but there are a few problems with it:


Your motherboard does not support SLI. In addition to a hardware bridge on top of the cards, SLI needs a motherboard that supports it too.
SLI requires that both cards are identical with the same chip.
To the furthest extent of my knowledge Mac OS does not support SLI.


If you want to upgrade, I know that people have used GTX 660s on the Mac Pro, but not anything after that.