Thank you very much for your guide! I have tried myself and following some Google Searches settings, but with no success. Well, I think it was of no success. But after applying your settings I saw that both GPUs are working now and I really needed that I have disabled the Vertical Sync though? I'm using G-Sync and this can screw the FPS. I have tested this for hour and more, but I'm still in one dungeon so I cannot guarantee.
For the other people to see my config, so they know that it can work for them:
* I'm using SLI (x2) GTX 1080.
* All is 4k: textures & monitor
* All is max details
* I use the Photo-Realistic ENB mod from Nexus.
* I use like 180 mods (not very heavy scripted) many of them textures and models.
I'm onto like 30 FPS outdoors (before this fix), and 40-50 and more FPS indoor with that fix. Of course, these are very context dependant. I have not yet tested well, but it looks like I have like 10 FPS on average increase. I'll be back to specify better.
Damn, I realize that above look like I'm showing off myself, but my intent was to know what kind of FPS you can expect in such setup and scale it to your solution.