I have 2 5850.
It seems like 1 card by itself gets better FPS then 2 graphics card at the same time, what is the issue here?
Is it supposed to do that?
Don't worry you don't have a hardware issue, its just that GW2 doesn't officially support multi-GPU Tech. I think AMD and Nvidia are trying to combat the situation at the moment. Just disable one of your cards when playng GW2 for now until a CAP comes from AMD supporting it.