So I created a post a few days back about a similar issue, but neither were resolved and I now have more information.
I own 2 GTX 660 Ti's, one a PNY 2GB model and the other a EVGA 3GB model. When I put them both in my motherboard, they are both recognized, they both work individually, but I get no SLI option in NvidiaControlPanel.
I have have fully wiped and updated both of there drivers to match, they both run at the same clock speeds BUT they have different BIOS versions. Other than having different Memory sizes, and different manufacturers, this is essentially the only difference - at least in GPU-Z - that I could find. I've re-arranged the cards and the configuration/direction of the SLI bridge but my efforts were to no avail.
I've found other forums with people with similar issues, but none of there's had been solved; even when they changed the SLI bridges. From what I can tell this is a very uncommon problem, so anyone have anymore solutions?