No you cannot. You would have to get another 6850 to crossfire them. You can crossfire different brands though. (MSI, GIGABYTE) etc..
100% wrong. you can cf any two cards with the same first two numbers in the name. it isnt ideal though as only some game support the use of both cards at full specs and even in the games that do you are likely to experience microstutter.
didnt i just answer this question a minute ago? why double post it wastes peoples time.
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).