There are a few possibilities here. I'll list a few:
1) Your CPU could be bottlenecking the system in that game. Adding Crossfire to the mix, forces the CPU to work a little harder causing a small decrease in performance.
2) Your CPU could be bottlenecking the system in that game. Adding a 2nd card put your PCIe slots in x8/x8 mode. Now you also lost a bit of performance due to the loss of PCIe lanes.
3) You have vsync on. Vsync prevents more than your monitors refresh rate, which is likely 60. You could power it by the worlds most powerful system, but it wouldn't matter, vsync prevents you from displaying more than your monitors refresh rate.
There are other possibilities...
With your current resolution, and FPS, you have no reason to have a 2nd 5850. If you plan to upgrade your monitor, it may be useful. What's the rest of your system like?