As of now in Rise of the Tomb Raider, the second GPUs are useless. But someone has found a fix for SLI with multiple NVIDIA GeForce video cards.
“Blaire” from 3D Center found some things when he was playing around with in the NVIDIA Inspector Tool for SLI, which enables awesome SLI results. In order to fix your SLI profile, 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.
If you not confident enough to do it yourself then you can you can import the profile by downloading it from here.
After you’ve done this, you will have 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.