That's a fun comparison, but can you run those 2 m5 pros in parallel to accomplish 2x the work? Otherwise, you just told me you can buy 2 toyota corollas for the price of 1 F-150 while trying to convince me you can haul your boat behind both corollas at the same time.
You can also buy a 64gb mini, save $1k and do more work than what you could do with a single 5090.
In Europe I can get a 128gb mac studio m4 max for 300 euros more than a 5090 (for which you still need to buy a power supply, motherboard, cpu , &c.)
Maybe not 2x (scaling is never linear) but you can absolutely chain them, and macOS supports RDMA over TB5 for even better performance https://news.ycombinator.com/item?id=46248644
Maybe hold back on the attitude