I am researching graphics solutions:
Connecting a theoretical Apple Mac Pro with Thunderbolt 3 to external GPU enclosures to take two AMD FirePro W9100 professional workstation graphics solutions & one or two NVIDIA NVS 810 multi display (digital signage) graphics solutions.
The network of graphics cards is pretty extreme for a Mac, but the two different GPU manufacturers will be the main issue. External GPUs are not hard to configure on the Mac Pro.
The AMD FirePro W9100 works well with the Mac Pro, which has two AMD FirePro W9000 GPUs internally.