I've heard nothing but bad things about running 2 video cards in SLI configuration with Wow. Most people experience performance losses, and I've actually yet to read a post where somebody had a performance gain with 2 video cards in SLI. Wow does not officially support SLI configurations.

On the flip side, perhaps if you ran the cards independently, out of SLI mode, and split your instances of Wow across the two you'd see a performance gain. I'd be interested in hearing the result on that.