So, after lurking awhile I decided to come out of hiding and see what everybody thought about this situation.
This is resolved, but I was a bit curious for future reference. When my aunt got married, my grandmother wanted to write a check for the happy couple (Aunt E and Uncle L), but she wasn't sure how to address it. She was going to do the standard 'Mr. and Mrs. Hisfirstname Hislastname', but Aunt E is a vet, and her official title is Doctor. (She also kept her last name, to avoid the hassle of changing her license.) Uncle L does not have a doctorate. She considered doing 'Dr. and Mrs. Hisfirstname Hislastname', but my mother talked her out of it. I suggested 'Dr. and Mr. Herfirstname Herlastname', but was told this was improper. In the end, she just wrote out each of their full names, without titles.
In a similar scenario, how would you proceed?