Setup updated with RTX 3090 as main graphic card. Unfortunately NVLink does not work in windoze so only 11GB memory available for Blender:
NVLink works great in linux Ubuntu 21.04 – 22GB available for rendering + 2GB for viewport/system (RTX 3090 24GB):
For full speed Ethernet 10Gb Mac <–> Linux connection you need to use crossed cable (for windoze it was not a problem to use non-crossed cable).