SLI only works in fullscreen.. if you aren't doing that, then do it.
I myself am running a 780 Only one, and i get 60 or 70 fps in vally benchmark.
well i play fullscreen and HEAVEN benchmark i go fullscreen. I thought i didnt have to state the obvious to say i have turned on the pc. I think its wierd to have to say i have SLI enabled and stuff like that.