My parents purchased two duble wides they have never bought a home before this?

My parents bought two double wide in 2016 before this they have never purchased any kind of home, they normally file separate but since they have purchased both double wide together; how do they claim one or both double wide can each claim one or only one on each separate return or only one on one return and nothing on the other return or only one home on a join return? They are married.