Same-guest reservations are merged under the same conversation thread in the Inbox to provide you with the context of past communication with that guest. Users can select which reservation to see from the drop-down at the top of the Reservation section of the Inbox widget, but the thread will display all of the messages, regardless of the reservation that was selected.
This could be problematic if the reservations differ in listing. A user with permission to one of the listings will have access to the entire conversation thread and therefore to the other reservations, even though they should not have access to them.
To prevent this, publish the listings to different Rentals United accounts. As long as the two reservations are on the same Rentals United account, they will be merged in the Inbox, and anyone with access to one of the reservations will have access to the other reservations as well.