thread-concurrency is only for scrypt mining right?
Yes. I was assuming scrypt because of the high memory clocks. For SHA256, 1650 MHz is a waste of energy. Set memory clocks to 300 MHz or the lowest setting your drivers permit.
Either card works well on it's own.
By "on its own", do you mean that you have only one card installed in the rig or that one is installed but inactive? Cards not connected to monitors can go into low performance mode in multi-card setups. Download GPU-Z and verify that the current core clocks of your cards are actually 1075 MHz.
Maybe your intensity is too high. With CGMiner, intensity 13 (20 is the highest setting) gives me the best performance. Start with 9 and increase one by one.
Getting about 450 - 500 hash from one card mining by itself, which is good for a 7950.
Well, it's moderate. 1075 MHz should give you about 560 Mhash/s, and you should be able to overclock the cards a little more.
In CGMIner, try diablo kernel, 1 thread per GPU, 1 vector and worksize 256.
If that fails, which Catalyst version are you using? Have you tried more than one? How are your temps?