Sir
I have had a similar situation (several years ago) and just to be helpful: in running dual 3090Ti I would suggest the NvLink bridge and dual 3090 Ti Founders Edition. While not obvious at first, the NvLink bridge made several memory issues/constraints go away for me. The bridge is not expensive an solved my problems.
Additionally, I had no issues running on Ubuntu with that configuration
(this is not gaming or SLI)
Lastly, keep an eye on your power requirement in running CPU, Ram, board, storage and 2 GPUs
I use two GPUs currently for what I do and since its my personal PC, its all funded by my own pockets so I understand your situation.
Respectfully