So I have two EVGA GTX 660 SC's, and I'm trying to stretch their performance a bit further by overclocking. I am using EVGA Precision to do this. Problem is, only my first GPU seems to be overclocked, even though Precision is saying they are both overclocked. I am using FurMark to test, and when I am NOT using the overclock profile, GPU 1 core is at 1047 MHz, and GPU 2 is at 950 MHz. When I OC, GPU 1 is at 1150 MHZ (obviously OC'd), but GPU 2 still reads 950 MHz. There are no power issues, I have a 750 watt PSU and without the OC I only use 400 watts (under load). I am using Gen3 link speed since I have an Ivy Bridge CPU, and my SLI configuration is 8x 8x. So why is GPU 2 not overclocking? The software recognizes both GPU's, and increases their fan speeds as well.
Furmark has issues with sli sometimes. Try running one of the free 3dmark benchmarks and see if will find the right settings for your oc sli. If not then try using msi afterburn to overclock the cards and see if that will keep the cards at the same speed.