* Informally means casual so you would address a husband and wife by their first names. Formally is when you would use Mr. Doe or Mrs. Doe.
As Mr & Mrs.
Mr & Mrs James Angstrom Jr.
Major and Mrs . . .
Dr. Mary Smith and Mr. James Doe
Just address the card to Mr and Mrs (husband's name). It's perfectly okay to address the husband and wife but to avoid offending the wife just do the Mr and Ms last name and you're probably safe.
How about Mr. and Mrs.
Pastors George and Martha Maloney
Governor and Mrs./Mr. Smith
* Major & Mrs. John Doe
Doctor Smith and his wife Reverend Smith (assuming she is using his last name).
You write “Dr and Mrs. Smith”
Pastors George and Martha Maloney