I'm trying Thea for cinema 4d r18 in OS X 10.12.6. I'm with a Nvidia 1070 and 980 TI (last drivers and cuda 8.0.90).
With only 980TI installed all is work, with 980TI plus 1070 device manager show all cards, cinema show only 1070 and it does not render with Gpu (only Cpu), with only 1070 it does not show any Gpu.
With only the 1070 I've try to delete presto-x64.cuda.dylib from the cinema 4d plug folder and rename presto-x64.cuda8.dylib removing the 8. After that, cinema see the 1070 but obviously it crash.
BUT Thea Studio show all the two cards and work perfectly with both (together or with only one of that installed).
I tried Thea for SketchUp (last version) only with 2 gpu and it crash