The answer is that single pass rendering should not substantially lessen SLI's effectiveness at offering performance gains over a one-card setup.
http://international.download.nvidia.com/geforce-com/international/pdfs/GeForce_GTX_1080_Whitepaper_FINAL.pdf...
Reading the Unity Release Schedule, it appears that VR Single Pass Rendering will make it into the next release, but VR SLI will not. Doesn't Single Pass rendering make the benefits that VR SLI would provide redundant?