Now that Rise of the Tomb Raider is out, people are finding out just how useless their second GPUs are - except now, someone has found a fix for SLI users with multiple NVIDIA GeForce video cards.
3D Center's "Blaire" found some things to play around with in the NVIDIA Inspector Tool for SLI, which enables awesome SLI results. In order to do it, you'll need to search for Rise of the Tomb Raider's profile, and then change the SLI bits (DX11) to 0x080002F5. After that, click the magnifier icon that will reveal NVIDIA's Undefined options, and search for 0x00A0694B and change it to 0x00000001.
After you've done this, you've enabled full SLI support for Rise of the Tomb Raider, with DSO Gaming reporting that they've noticed 95% scaling on their GeForce GTX 690 - a damn good result for SLI scaling.