If the wife has taken his surname then "Dr & Mrs James King".... or possibly "Dr James & Mrs Susan King".
If she has retained her maiden name then "Dr James King & Ms Susan Jones".
Mr. and Dr. Joseph Smith (indicates the wife is the doctor)
If it is a formal letter, "Dr." or "Mrs." should be used.
Dr. and Mr. Steve Smith
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.
Dear Mr. (name) and Mrs (name)
"To Mr. and Dr. Smith." Or just separate the names, "To Mr. John Smith and Dr. Mary Smith." If it is a casual letter you could simply write "To The Smiths" I am guessing the original form of address you want to modify is "To Dr. and Mrs. John A Smith." But, this is an increasingly archaic form of address and is not applicable in the case of the wife being the doctor or professor nor can it be used when the wife has not taken her husband's name.
If you're addressing it to both of them - It would be: Mr & Doctor Smith OR Mr & Mrs Smith - depending on how well you know them.
Example : Mr & Mrs Edward J. Flynn Jr.
What is the correct forma when signing a letter from s Sr. and his wife, to distinguish from Jr. and his wife?
The address should read: Drs. John and Jane Doe or, Dr. John Doe and Dr. Jane Doe.
Doctor Smith and his wife Reverend Smith (assuming she is using his last name).
You can address a retired doctor and his wife as "Dr. and Mrs. (Last Name)" when speaking to them formally. If you are on more familiar terms, you could simply address them by their first names.