I'm going to do it. I have 2 ROG Thor 850's and (individually) they aren't enough for the 3090 xtreme ... Too much draw, system reboots (the display shows over 865 sometimes, kinda impressive) so I'm jumping the second to use just for the GPU. Also will be able to read out the cards usage in real time... The power on pin just needs a comm pin ... There's nothing with load being messed with so I think everything should be fine.
I'll report back... If I remember