As of version 4.15, Unreal Engine supports NVIDIA's Alternate Frame Rendering (AFR) for packaged games running on NVIDIA SLI configurations. AFR works by alternating frames between the linked
GPUs; one frame is rendered by GPU 1, then the next frame is rendered by GPU 2, then back to GPU 1, and so on. By using multiple GPUs on a single monitor, its output can increase in image quality
Developers who intend to use AFR will still need to work directly with NVIDIA to test their games and to have it automatically switch over to use the Alternating Frame Rendering functionality
For additional details, you can read NVIDIA's documentation on their SLI modes, specifically Alternate Frame Rendering here.