Can't overclock GTX 660 SLI

[Sewdle]

I have two GTX 660s in SLI from different manufacturers and both are already factory overclocked but I can't overclock them even the slightest, I've tried all overclocking software that I could find and none of them work. Is this because they're already factory OCed? Anyway, a solution to this "problem" would be much appreciated.

 

[dantethedemonkiller]

It's because it's already over clocked and though sli beats crossfire of and it still got some limitations