I went through this initially when adding a second 7800GT to my rig - the second card wouldn't detect (and therefore didn't show up in Device Manger), then after endless reboots did detect, but wouldn't allow me to enable SLI with the same "remove incompatible adapters" message you're getting.
Its hard to remember but I seem to recall removing one adapter, resetting the SLI flip-card to single card mode, booting back up, shutting down, flipping back to dual card mode, booting up, shutting back down, then adding the second card.
Then it was all gold. I've done a rebuild since when I moved to the raptors and have had no problems on the new install. I think its just old installs that had a single card that get freaked when a new one is added.
EDIT - I wouldn't trust the GPU load balancing info very much, BTW
In SIN Episodes I get just under double the framerate with SLI enabled but GPU load balancing always gives one card the bottom 5% of the screen and the other card the remaining 95%. I don't think its particularly (or at all?) accurate.