There is no SLI finger for the 1060, so its "physically" not possible. Some did SLI through DX 12 (Ashes in singularity test), and in theory it should be about equal to the 1080. 2 x 1060s have same shader count, lower bus, but more ROPs, so it all depends on the game/bench tested.
http://www.gamersnexus.net/guides/2519-gtx-1060-sli-benchmark-in-ashes-multi-gpu
TLDR; its not supported and cant be done on a hardware/nvidia-software base, Dx12 can do it though...