Yes, there should be no issue as long as they both work OK by themselves. You will probably want to uninstall the gfx driver/re-install it when both cards are in place. What MB will these be going into?
Sweet, I have read a couple of other forums and they say the same thing...
MB- Asus Crosshair V Formula
CPU- AMD FX-8120 OC to 3.8- 4.2 GHz
GPU- Radeon HD 6970 diamond edition
RAM- 16 GIGs cossair "1333"