Ok so I removed everything and installed 11.11 + profiles (i was at 11.3 before), and when I went in, Xfire was enabled from the start, and Furmark reported both to be working when doing a burn in test. I cooled my cards down to 30C, then turned my fans down to 20% and let it run. Before I was only geting 8 fps with stock gpu settings, 1680x1050 res, 8x msaa. I noticed that before, only 1 card would get hotter. when I ran it this time, they both got hotter.
I tested this out in Portal 2(Legit version from steam) with the following settings:
Very high detail/memory pool
8xMSAA 16x Anisotropic Filtering
Multicore Rendering enabled
3x buffered V-sync
And got a constant 60fps with no tearing, artifacts, anything. I then turned v-sync off, and got 300fps, with the same results as before: no tearing, artifacts, anything.
What game(s) are you trying to play? I know that some games don't play well with X-fire...