Care to explain which 7990s, and what your Motherboard slot setup is? Do you have 2 free slots between your PCIEs?

Also it might not hurt to put a custom cooler on the first 7990 as that one will get significantly hotter. I did not mine with 7 series, but I did do it with the 5 series back on a slower board with only 1 free slot in between. The lower card could tolerate far more OC and there was an easy 10C if not more difference.