Yes, iNat documents interactions between individuals (humans) and nature, so each one human/nature combination is a valid interaction as long as observers are using their own photos and abiding by other rules.
It’s not-optimal for other reasons (essentially duplicate data, more work for IDers, etc.). I think most observers “grow out” of this (except for perhaps very rare/cool observations that people want on their lists). This behavior is most often seen in school classes or other groups that are being trained how to use iNat.
I have seen moth images in which the person adds in the note - ‘also seen by x’ sometimes ‘x and y’. I presume they were on a collecting trip together. I’ve only seen this with experiences observers, though.
I’ve seen a case where it was very helpful for IDing both because the pictures captured from people standing at different angles and therefore caught different features.