Ensure your PSU has the wattage to support it, and the 6+2 pins or whatever the card requires. Plug in, reinstall Drivers (use clean installation), apply your SLI Bridge, and go into Physx settings in nvidia manager and enable SLI.
Ensure your MoBo is SLI Compatible, and make sure you put the two cards in SLI compatible PCIE slots. My mobo has 4 PCIE slots, but only two of them support SLI.
"So many times I wanted to move about (make poop but can't leave the game because it's online)" - Underworld