If your system is running OK no-sli with both cards installed they should work in sli and we can help get you there, you will have one caveat, with your Intel 750 drive installed your video cards will run at PCIE 3.0X16 and X8, both will not run at X16, first what slats are your GPU's and Intel 750 installed
Video cards in pcieX16 slots 1 and 3, Intel 750, slot #4
Both video cards must be showing if you can select sli so I assume both cards are working but to test 1 card at a time connect monitor to top card, disable second card using pcie lane switches at top right of motherboard, run some 3D tests, either benchmarks or games, then move monitor cable to second card and disable top card using pcie lane switches and test the second card, make sure computer is turned off when disabling/enabling pcie lane switches
If both video cards function as intended enable both cards, install sli bridge, need HB bridge for sli on Pascal
enable sli and wait, it may take several minutes of the screen flashing on and off, a courser may appear but eventually screen will come on normal and sli will function, I was benching 4 Pascal cards and I gave up myself, then I went outside and smoked a cigarette and came back inside to see my desktop, i don't suggest smoking a cigarette, just walk away for a couple minutes
Please let us know how you are getting along