A domain could indeed have multiple PTR records and they can exist with other records for a name, however for PTR records to be used for the purpose of aliasing

A domain could indeed have multiple PTR records and they can exist with other records for a name, however for PTR records to be used for the purpose of aliasing there would need to be some sort of additional processing in resolvers. If I were to write an RFC then PTR records might be a good place to start. Alternatively I think NAPTR records could be used as well.