This may be a really silly question, but I currently have a GTX 780 in my PC and was about to buy another to SLI.

HOWEVER, I've read somewhere before that Skyrim will only use one GPU, is this right or will SLIing with 2 of these cards boost performance?

SLI is not a magic panacea for game performance. Some games benefit from SLI, most do not. Of the ones that do, it is at the higher resolutions, like 2560 (or whatever it is) that see the most improvement.

As for this game in particular, no real benefit to SLI on most systems. Your mileage may vary.

Indeed, depends on the game in question. Skyrim however doesn't by default require loads of GPU power, the amount of VRAM is more important. Oh, on thing about VRAM is important to remember: in SLI (as well in CF) 4GB + 4GB is still 4GB.

Accordingly to that Skyrim supports an Nvidia SLI configuration.

http://www.game-debate.com/news/?news=10694

Also you can see benchmarks with a GTX 680 single GPU, 2-way until 4-way SLI system.

http://nl.hardware.info/reviews/2641/10/nvidia-geforce-gtx-680-quad-sli-review-english-version-the-elder-scrolls-v-skyrim

So, you can judge yourself. However, i saw people without any significant problems running Skyrim with an ENB, and some other with low fps performance.

In some case the main cause was mod or driver related. !!!