I don't think it's that easy on a laptop - no second PCI slot - no sli bridge.
It is rumoured that there are a few high end sli and crossfire-x cards such as the GTX 680m sli, 675m sli, HD 7970m-CF. These cards are top$$ and have high power requirements so that would have to be checked before purchasing and almost all information I can find about them seems somewhat in conflict - here is a listing of mobile GPU's "available"http://www.notebookcheck.net/Comparison-of-Laptop-Graphics-Cards.130.0.html
... if you can find any of those cards (finding new laptop parts is tricky at best). I could find no hardward specific (which I think would be needed) info about mobile sli/x-fire cards either.