I just set up a 1950x machine with my 1080 Ti's. Anecdotally, I've found that there's a balancing act to ensure your CPU projects don't starve your GPU projects. Even though lots of projects like to say "0.1 CPU + GPU" or things like that, I've found GPU projects seem to need a lot more CPU than that. For reference, I can run 2 GPU SETI tasks, plus about 20 World Community Grid CPU tasks with none of them stepping on each other's toes. It's a Windows 10 box, and when I monitor CPU usage, it generally hovers around 70%. If I start to push things and add more CPU threads, that's when it starts slowing down other projects. But again, I haven't rigorously measured any of this, so it's possible there are combinations of projects out there that perform better. I'm just too lazy to do the min-maxing. :)