No the cards have to be in the same model number.
And the amount of memory of each needs to match.
You could use the 7950 card if the option is available to set it to do the dedicate Physics calculations, taking the load off the 7970 card doing it as well as the game graphics.
But you will not be able to crossfire them.
Practically because the base gpu core speeds differ at stock, the memory speed, and shader unit speeds.
and the number of shades each card has.