The Dell UP3218K monitor I describe does not work “out of the box” on any OS. Even finding a GPU that can drive it at all is tricky.
Then it should show up as single display.
It's also how Apple XDR display presents itself to MacOS (two DisplayPort 1.4 tunnels over USB4, tiled layout in DisplayID).
I suspect it's possible that it doesn't have a valid tiled geometry block, but that's something that was already handled right when first 4k displays landed, so...