I found the same thing. After experimenting a little with World of Warcraft (which is pretty much the only game I want to run a panoramic monitor with), I found that on a single monitor, I got from 34 to 90 fps with one card, and a stuttery 12-30 fps with CrossfireX.
Now, I always thought that, with graphics cards as with girlfriends, two at a time is better than one. Guess I'm wrong. I am currently using only an Asus 5850 Top; my Sapphire 5850 (slightly lower speed, considerably better English-language documentation and support) lies fallow.
On my Gigabyte motherboard, the SATA ports interfere with the second graphics card anyway. I went to angled connectors, but it's still too tight for comfort. I'm using one card to drive two monitrs at the moment, and hope to go to three (5040x1050) in the near future. But unless someone can think of a reason why enabling CrossfireX, even with one monitor, would decrease the frame rate, I'm sticking to the single card.
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