I always look at individual cores, and it's generally balanced relatively well among the first 4, none of which exceeding 50%. Maybe the sticker said i7 2600k, but the processor in side is not, if you are getting capped cpu, which you never mentioned, merely saying that it must be the cpu if your framerate doesn't chagne. Have you looked at the cpu when comparing single vs crossfire? Perhaps you don't have a multi-gpu profile loaded? Are both cards being loaded when you play in crossfire? do you have gpu monitoring that tells you how heavily they're being used?