When creating letters, you can combine spouse pairs so that they only receive one letter by checking the box Combine labels for each spouse pair on the details tab of a letter parameter. If the primary addressee defined on the spouse records are different, the system will take the primary addressee of the record that is listed first on the student's relationship tab.
We resolved this issue in version 7.82.104, patch 14. Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.