As long as they are the same clock speeds, then you should be fine. Otherwise one will bottle-neck the other.
Even if they arent the same clocks out of the box, you can change them to be through Catalyst Control Center.
EDIT: I am actually crossfiring 2 HIS Radeon HD 6870's, one is turbo, and the other isnt, so I just overclocked the 2nd one to the stock speed of the turbo, works like a charm! =D