For OCing tests, use Furmark, Heaven, or any other program.. I use Furmark to test my OCs on my cards..
2 identical cards never OC the same, so maybe doing it on CFx is causing the crashes, so like Vantharas told you, test one by one..
Use MSI AfterBurner to do the OC, like this: move the slider to increase, let's say, 5-10Mhz, save, and run Furmark (or the program you have chosen).. If artifacts/blue screens shows up, that's the limit and you have to step back.. Of course, this is on each card, not CFx..
Sorry can't be more helpful on the CFx OCing, but I only have one card, and always have.. But for the OC, sure..
8320 @ 4,7 / 990FXA UD5 / CF-X 7970 + 7950 / Switch 810 / H110 p-p / 16 GB RAM / 23" 5760x1080 / SSD 64GB RAID0 / HX750 / SB Recon3D Fatal1ty Champion / z506