Your average FPS is so close to that of the most common monitor refresh rate (60Hz) that I wonder if maybe VSync is what's slowing you up. With VSync enabled, the GPU's keep their FPS below the monitor's refresh rate as anything over can cause tearing and is essentially wasted. VSync also has the added affect of lowering power consumption and heat production because it only allows the cards to work hard enough to pull off 60FPS, nothing more.
Considering your dual GPU usage is about half that of your single card usage, I'd imagine that's what's going on. Try a benchmark that is more graphically intensive (FurMark or Unigine Heaven come to mind) or turn off VSync. I'm sure you'll find your results start to make more sense.