Yes you can, but it's not ideal. One card will render frames faster than the other so a 6870 would be limited by the slower 6850. Worst case it's the same as having two 6850s, though in some cases it would be between two 6850s and two 6870s. It's best to run two 6850s though. Having one card run significantly faster that the other can produce more noticeable microstutter (saw this with my 3850s initially when the second one came OC'd a good bit till I OC'd the slower one to better match).