If both tenants are on the same lease to rent the same unit (house, apartment) then the best thing for the landlord to do is nothing. Presumably, either one or both of the co-tenants will just move out because the living arrangement is no longer suitable. If that happens they will either give proper notice in writing, ask to assign the unit to someone else entirely, or they may both leave abruptly. If only one of them wants to leave, they are both responsible for the obligations they signed on for UNTIL they are both out of the unit. They both have the same rights and the same obligations to the tenancy. The Act does not really allow for partial assignments. In the meantime, ideally you should not be involved in any process or discussion that involves making current occupants a party to the lease until/unless both of the original tenants are in agreement about the matter. Also, unfortunately the Residential Tenancies Act does not ensure or enforce a landlord’s “right to know” who else may be in the rental unit with the tenant(s).