Anecdotally, I don't know too many people who share 1 Switch because of how impractical it is to share a handheld.
The biggest issue here is one of communication, I feel. The primary player should've had a more serious title and explicit role, like founder as I suggested above. Or, a more popular decision would be to not let people share islands, but give each account its own island and save
Maybe not on this forum, but switch sharing is pretty common. I don't really play it much undocked and for that reason it is really just another console in my household. I know it can be used in handheld form but Nintendo didn't really sell it as a handhold only device. I think it would have made more sense to maybe make the second player have their own island right next to the first players island and you could easily boat across or something. There are a lot of ways Nintendo could have made the game more local co-op friendly but they chose not to. I think that is my biggest issue with Nintendo, a lot of their long running franchise don't innovate very much sequel to sequel. They certainly do add new features, but not nearly as much as we expect from other 1st part developers in subsequent sequels of a franchise.