Well do you need more computational power or more graphical power :/ it's really not for anyone else to say. Personally i'd get 2x 1080Ti and a 6700k/7700k
I wouldn't invest in another gpu, sli is being phased out. Few new games actually support it. Not worth the $700-800usd price tag, would be worth it to take that money and get the 8700k and new mobo
People SLI because they can, it doesn't help very much anymore and some games actually see less frames with 2 cards. I have 2 1080tis in my PC and I disabled SLI because it was a pain in the ass. The second card is just for mining now