I was asking because in the latest VRWorks for UE4.12, there is VR SLI.
It seems Nvidia Funhouse (made with UE4) can use SLI, they recommend to use dual GTX 1080 for the best experience:
“If you’ve got two identical GPUs and a GeForce GTX 980 Ti, or higher, dedicated to , VR Funhouse will activate VR SLI, enabling GPU 1 to render to the left eye in the VR headset and GPU 2 to the right eye. improves performance while keeping latency low, further improving the experience.”