If the cards are different speeds, and you want them both to run as fast as the fast one does, you can flash the firmware of the slow card to make it as fast as the other. You'd want to test it in single card mode at that speed for a while first, obviously. If it gets too hot you'll get problems.
I lost some monitoring abilities when I mixed two cards of different make together. One card didnt have all the same stuff as the other.