Option 1: Create two separate mailings so you can define different Name format options
Process: You will need to create two different selection queries to break-out the mailings. You can use this query for constituents with the same last name and this one for constituents with different last names by either adding them as an additional filter or using a separate selection query with those fields. After that you can create two mailings within an Appeal and set different Name format options for each mailing to get the right Addressee fields.
Option 2: Use a single mailing and output the information you need so you can make manual corrections in Excel.
Process: You will want to ensure you add the Spouse last name and Spouse full name fields as output fields in your mailing. You can then use this method in Excel to speed up the process of manually correcting the records.
For more information, see our idea on the Altru Community