maybe your running Furmark at a low resolution and your CPU is bottlenecking the performance. Just an example of what I noticed in RE5 (Resident evil 5)
Core i7 920 @ Stock 5850@ 1000/1200 All high 8xAA 1280 x 768
Core i7 920 @ 4Ghz 5850@ 1000/1200 All high 8xAA 1280 x 768
Thats a 26.9% increase. With your two cards the bottleneck will start at higher resolutions. What I suggest you do is set Furmark to your highest resolution and test with one card, then test with crossfire and compare the results.