One of two owners in common can mortgage their interest in property. Of course, a lender does not want to go to court to order a partition and sale of property, so the lender will not accept less than a security interest in the whole property. When a married person mortgages property held by the entireties, there is no effect since the married couple is viewed as a single entity (except when it comes to tax liens - see the Supreme Court decision in US v. Craft).