Dear Mr. (name) and Mrs (name)
Senior Doctor first if it is a medical reason or unmarried, Man first if it is a married couple.
Same as to anybody else. Put the name and address of the person clearly in the center of the envelope.
Address on envelope to a doctor: Dr. John Doe is the general form.
The correct way to address the envelope is: Mr. John Doe & Dr. Jane Doe.
Senator John Doe, MD
A PhD is a doctor. You address a letter to him Dr. John Smith. If he is married you address it Dr. John and Jane Smith. If she has the PhD, you address it John and Dr Sue Smith. If both have their PhD's you address it Drs, Henry and Jane Smith.
Whichever is the highest qualification, which would be a doctor as a mayor is elected not qualified.
When address to a doctor it should appear as: Dr. John Doe or John Doe, MD.
When addressing an envelope to the wife of a doctor who is not a doctor herself then and he is Dr. John Doe, then address the envelope to her as Mrs. John Doe or Ms. (Mrs.) Jane Doe.
The proper way to address an envelope to a doctor office is to first input the name of the clinic. On the second line you can insert in care of or C/O with the doctor's name. On the third and fourth line you can insert the address, city, and zip code. Include any suite or floor numbers of the doctor's office on the third line.
That would be: "Dr. John and Mrs. Jane Whomsoever".