When I worked at the DMV, one thing I learned very early when working with vehicle titles (i.e. deeds of ownership, but for vehicles) with multiple names on them is to pay careful attention to the conjunction between the names.

"Alice and Bob" is a different kind of ownership to "Alice or Bob". And meant that, to do anything with the title, you needed consent from everyone involved. Or, on the other hand, meant that any person on the title could do basically anything they wanted with it with only their own consent.

The conjunction was set up at the time the title was issued, and by filling out the application with a particular conjunction selected, you were giving your consent to play by the rules of that conjunction. So, by signing a form with an "or" conjunction between you and someone else, you are implicitly giving permission to the other person to give your consent for you. If that isn't what you want, that's what "and" is for.

Point being, in systems where robust rules are needed, they can be made. This is really just the basis of how all rules and agreements are created. The abstract concept of "ownership" is no different. It can be whatever you define it to mean.