Depends a lot on the game, the second card is turned off under normal loads.
I can max out skyrim now with all the high texture mods and maintain a good 45+ fps.
I got about a 65% improvement in my furmark score.
FWIW I was really concerned about cooling of the top card which interestingly enough hasn't been a problem.
All and all I will probably continue playing with it for a week or so then sell it as I don't really need that much fire power.
It's also worth noting, GPUs for some reason are crashing recently.
I got both of my 6970s for under $250! (The first one was 130 the second was 118) Which is just nuts considering what they used to go for.