HR - Married Employees who are Contacts

We have a married couple who are each other’s contacts, i.e. the husband is the wife's contact and the wife is listed as a contact on the husband's record. The husband's SSN was not populated back in 2004 when he was added as a contact. He has an SSN on his employee record. When we try to update his contact record on his wife's record with his SSN we get a message stating it's a duplicate SSN. He termed in 2005. I tried to date track to 2005 and delete his contact record but I received the APP-BEN-94121 error message.

Any ideas on how  we can add his SSN to his wife’s contact record?