Premiere does not use or care about SLI.
6K timeline work can be a bit taxing and about a year ago (can't vouch for the latest CC2015 version) I actually had the best timeline experience on 6K with two cards, but only one configured to be used by Adobe - one was hooked up to a single 4K monitor and the other was configured to be used for Premiere Pro. With both cards configured to be used for Premiere Pro, I was getting some dropped frames, and when I move to the one on / one off configuration playback of full res. 6K was working with zero dropped frames.
In my case, I was using a pair of GTX Titans, but I don't see why this couldn't work in your situation with two dissimilar GTX cards. I say GTX cards so that both cards are using the same video drivers.
I have a pair of GTX 1060's that do work perfectly together. Unless you really are a heavy user of MPE accelerated effects and features you could be disappointed in multiple GPU's improving day to day performance. My forte is benchmarking Premiere Pro, so I occasionaly do go to extremes. I love the new GTX 1060 especially my model which is initially factory superclocked and easily and safely overclockable for Premiere CUDA applications. I will later be coming up with facts and figures but nVidia keeps releasing new drivers and I have to check if they make changes. Like going from 368.xx to 372.xx the newer drivers were faster than the old ones so my numbers have changed and this make comparisons hard to draw