#1) The rule is either ‘Dr.‘ before, or the post-nominal abbreviation for their degree after. Never both at the same time. 
#2) See the post below on joint forms of address — addressing a physician and spouse.
Robert Hickey author of “Honor & Respect”
These forms work for anyone with a doctorate as well as physicians such as dentist, chiropractor, military doctor, veterinarian, optometrist, osteopath or podiatrist.
Avoid: Dr. John and Ms. Kathleen Dexter 
Avoid: Dr. Allyson and Mr. William Carley
#1) Same Surname
 —- Envelope & Salutation: 
 ——– Dr. and Mrs. John Dexter 
 ——– (Address) 
 ——– —- Dear Dr. and Mrs. Dexter, 
 ——– Dr. John Dexter and Ms. Kathleen Dexter 
 ——– (Address) 
 ——– —- Dear Dr. and Ms. Dexter 
 ——– Dr. Allyson Carley and Mr. William Carley 
 ——– (Address) 
 ——– —- Dear Dr. Carley and Mr. Carley 
#2) Different Surnames
 —- Envelope & Salutation: 
 ——– Dr. Roger Fry and Ms. Jane Taylor 
 ——– (Address) 
 ——– —- Dear Dr. Fry and Ms. Taylor, 
 ——– Dr. Lucy Khin and Mr. David Patel 
 ——– (Address) 
 ——– —- Dear Dr. Khin and Mr. Patel 

My friend is a retired physician who no longer practices. His still Dr. (Name)? Does he still put MD after his name? 
 —————————— — Linda Whedbee
Dear Ms. Whedbee: 
Physicians are addressed as Dr. … forever … in practice and when retired.
(Full Name), M.D. is the official form of his name. He used that when practicing and including his degree made his qualifications to offer a professional service clear. He might still use (Full Name), M.D. in a academic setting where everyone is using their academic post-nominals.
Dr. William Smith is the social form of his name. Now that he’s retired it’s the version to use most often.
 —- Envelope: How to Address a Medical Doctor 
 ——– Dr. William Smith 
 ——– (Address)
 —- Converation or salutation: 
 ——– Dr. Smith
— Robert Hickey How to Address a Medical Doctor
Robert Hickey author of “Honor & Respect”
You can use these forms of address for any mode of communication: addressing a letter, invitation, card or Email. (If there are differences between the official and social forms of address, I will have mentioned the different forms.) The form noted in the salutation is the same form you say when you say their name in conversation or when you greet them. 
___ What I don’t cover on this site are many things I do cover in my book: all the rules of forms of address, about names, international titles, precedence, complimentary closes, details on invitations, place cards, all sorts of introductions , etc. I hope you’ll get a copy of the book if you’d like the further detail.
—- #1) At right on desktops , at the bottom of every page on tablets and phones , is a list of all the offices, officials & topics covered on the site.
—- #2) If you don’t see the official you seek included or your question answered send me an e-mail. I am pretty fast at sending a reply: usually the next day or so (unless I am traveling.) Note: I don’t have mailing or Email addresses for any of the officials and I don’t keep track of offices that exist only in history books.
—- #3) If I think your question is of interest to others, Sometimes I post the question – but always change all the specifics.