What if there is a dispute over control of the keys (possession)? That entitlement would need to be enforced, which could only be accomplished with a property rule (see Morris, Structure of Entitlements, 78 Cornell L. Review 822 (1993)). Moreover, the issue of multiple ownership can be analogized to a tenancy in common, which allows parties to own and interest in the undivided whole, thus enjoying the same property entitlement protections.