Computer graphics applications get no benefit from SLI, and the only benefit you will get from a 2nd card is when using a GPU render engine that supports multiple cards. Let's use Blender's Cycles engine as an example. The 750Ti would be ok to use as your display while letting your 1060 handle the rendering. But it could limit the overall rendering because the card with the smallest amount of RAM sets the maximum amount of RAM the GPU render engine can use, for all of them. So, in that sense the chain is only as strong as it's weakest link. That is why it's always best to use cards that are the same. Such as a 1080Ti w/ another 1080ti. Since the 1080 and 1070 both have 8GB of VRAM, you could pair both of them together, no problem. The 1070 is largely a 1080 with fewer CUDA cores.