Mont. Code Ann. § 70-26-101

A person who hires part of a room for a dwelling is entitled to the whole of the room, notwithstanding any agreement to the contrary, and if a landlord lets a room as a dwelling for more than one family, the person to whom the landlord first lets any part of it is entitled to the possession of the whole room for the term agreed upon, and every tenant in the building under the same landlord is relieved from all obligation to pay rent to the landlord while the double letting of any room continues.