HD 6950 Crossfire Issues

So I today i received another XFX HD 6950 DD from scan for a crossfirex config with my existing card. My power supply is a TX750 and so provides enough power to the cards. However, after I set up the cards and installed all the necessary divers (and switched it to crossfire mode in CCC), I played some BF3 to try it out and the fps was no different, or maybe slightly worse than if I was running just the one card. Please help as I hope I haven't just wasted £160 in another GPU.

Regards
Ollie