Thank you!
I had the same issue where my second graphics card was not showing up in my device manager and SLI option is not in the nVidia control panel.
specs:
Asus M4A89GTD PRO/USB3
MSI Cyclone GTX460 (geforce gtx 460)
Zotac GTX460 (geforce gtx 460)
nVidia driver 310.70
Issue was...