Update you nVidia drivers to the latest, and check Whitcher 2 benchmarks in SLI but i guess it's high than what you're saying, it's like the game is not benefiting from SLI or you probably have some failure with your SLI.
What's your gaming resolution ? and how many FPS you get in Crysis 2, Metro 2033, any DX11 game ?
All of my drivers have been updated to their most recent versions. My resolution is set at 1920 x 1080. I do not own any of those games mentioned above. This is my first SLI configuration, and now I'm starting to see why some say one big card FTW.