Since they are of the same GPU family, they should work just fine in CrossfireX mode. Different card manufacturers shouldn't matter. What power supply are you using?
Sorry, I'm running out of ideas here. Assuming you have enough power available, your computer should be running in CrossfireX mode. In fact, when I set up my 7770s for CrossfireX, Catalyst Control Center popped up asking if I wanted to enable CrossfireX mode. I just can't figure out why your setup isn't doing the same.