You can CF both of them. Which ever card is lower the higher card will drop to those settings. How ever it is aways a good idea to use exacted cards and according to AMD they don't recomend doing it.
But you can in fact on AMD cards a rule of thumb is the first two numbers have to be the same for example 6850 will CF with a 6870 and 6950 will CF with 6950 but you 6970 will now run as 6950.
Good luck to you