Strange, strange problem this one.
I had cycles with these 2 GPU + CPU working well for renders. Then, after some time, I took the 750ti out and continued working with only the 1070ti + CPU until after some time, I put the 750ti back in and this problem appears.
what I tried already:
-install new gpu drivers
-try with experimental versions (inclusing 2.8)
The scene you see (my startup file), was rendering and previewing also before i took out the 750ti out the system