GPUs aren't electrically isolated from the motherboard though. An entire computer is a single unified power domain.

The only place where there's isolation is stuff like USB ports to avoid dangerous ground loop currents.

That said I believe the PSU itself provides full isolation and won't backfeed so using two on separate circuits should (maybe?) be safe. Although if one circuit tripped the other PSU would immediately be way over capacity. Hopefully that doesn't cause an extended brownout before the second one disables itself.