It appears that NVidia has dropped support for multiple GPUs in OS higher than 10.13.2
Under 10.13.2 I had a GTX680 in my MacPro 5,1 and two Titan Xp in a Cubix Xpander. Since upgrading to 10.13.6 I started getting really bad window redraws on the desktop and just bad overall computer sluggishness.
With only a single GPU in the MacPro the NVidia Web Drivers work perfectly and I get great performance, but it’s a shame I can’t harness multiple GPUs like I did just 4 months ago.
I urge NVidia to not abandon us Mac users. Just because Apple went with your competitor doesn’t mean we’re not loving and relying your excellent products.