There seems to be some slight confusion with the state of SLI in Just Cause 3. In the PC Perfromance Guide posted by NVIDIA, and employee, Andrew Burnes responded to a question and said that the engine doesn't support SLI, meaning that we wouldn't be getting SLI at all.

SLI is possible in all games, and is engine agnostic.

That information is actually technically false. SLI is something that's implemented at the driver level and is compatible with every gaming application (DirectX or OpenGL) as stated in NVIDIA's own FAQ. It is engine agnostic and games only need proper profiles created for them in addition to some work in the engine so as to take advantage of multiple GPU's.

The impetus, of course, is on both NVIDIA and the game developer to work in concert to optimize this profile, and the engine, so that it actually scales and works well. But the point is that every graphics engine can make use of SLI. That doesn't mean it's an easy task to implement, just that it's technically possible. A lot of work does need to be done in order to optimize cross-frame dependencies, temporal effects, compute and physics data that shares frames and a host of other things to make it work properly and work well. AFR rendering specifically is a driver level feature. The game only sees one logical GPU, where the splitting and rendering operations are done at the driver itself, there is no intervention on the engine itself, though game engines do need to be made to play nice with the driver to work at its best.

The newest driver, 359.06, actually lists a profile added for Just Case 3, though it's a "single" SLI profile, this could be a placeholder as the actual profile is perfected, or it could mean another type of SLI (there are five in total) rendering mode is being used.

What this might mean is that either Hybrid SLI Rendering or SLIAA, where AA effects are offloaded onto the other GPU, is being used. Currently only Single-GPU is supported via the program settings, however AFR can be forced and it likely will result in increased performance, as this game engine is compatible as other games based upon it can attest to.

So no, the SLI profile is not perfect as of yet, but it is possible and a profile is something that NVIDIA has to create and put into the next driver release. To say that the engine is incompatible with Multi-GPU solutions is wrong. The individual in question likely misspoke before actually researching the issue.