Noah’s right, SLI isn’t supported in OS X, so strapping two video cards together won’t be possible until Apple makes those changes at the operating system level.
As far as using two video cards with FCP, that’s difficult as Final Cut often addresses the GPU directly and this causes problems when you have more than one video card installed. Apple’s Kbase article on it is here:
https://support.apple.com/kb/TS1828
To avoid problems, the current wisdom is to get the best single video card you can afford. If you want an Nvidia that would be the GTX285, or if you have the cash, a Quadro 4800. There have been some problems with Nvidia drivers in the recent past though, and people have reported Nvidia cards under-perform in Macs when compared to PCs. Nvidia is working on this, though.
Since you are using FCS, a really good option with awesome performance, and minimal issues is the Radeon 4870. If I was spec’ing out anything mission-critical, this is the card I would go with as ATI offers great OS X driver support, it will work in any Mac Pro, and it absolutely smokes if you’re a heavy user of Color or Motion. A good recent comparison is available here:
https://www.barefeats.com/nehal14.html
Also, do not confuse video game performance with performance in FCS; they are not the same. If you take a look at the article above, while the 4870 is quite a bit slower in Call of Duty, it is absolutely toe-to-toe with both the GTX285 and the Quadro (an $1800 card) in the Motion benchmarks.
One last tip: if you can, use a single larger display, rather than two smaller displays, as two displays will effectively cut the available video ram to each display in half, which isn’t good as FCS leverages vram heavily.
JM