How do you address a letter to multiple recipients one male and one female?

If they are married and share the same surname it would be "Mr. & Mrs. Initial. Surname." (I.e. Joe and Margaret White would be Mr. and Mrs. J. White.)

If they are married, but with different surnames or not married it would be "Mr. Initial. Surname. & Ms. Initial. Surname. (I.e. Mark Chase and Emily Brown would be Mr. M. Chase and Ms. E. Brown.

If only the male is a doctor, use "Dr." instead of "Mr." according to the above rules.

If only the female is a doctor use "Dr." instead of "Mrs." or "Ms." depending on whether form 1 or 2 is used.

If both are doctors, use this form instead of form 1: "Drs. Initial. Surname". In form 2, replace both "Mr." and "Ms." with "Dr."

Any additional letters after someone's name such as "Phd." "Esq" and the like should go directly after the surname of the relevant person