Dr. (First name)(Last name)
OR
Dr. (Last name)
For example:
Dr. Lily Copper / Dr. Cooper
Mr. John Doe & Dr. Mary Smith (or the reverse, depending on priority re content)
Addressing an envelope to a male child use to be 'Master', but now it can be Mr.and the female Miss.
* Addressing a female doctor and her husband: Mr. John Doe and Dr. Jane Doe.
Married or not , this person may be rightly addressed as Dr.
* The males name appears first. Example: Mr. & Mrs. John Doe
Doctors use the title "doctor" in Hebrew, whether they are male or female. Doctor is spelled דוקטור.
"Dear Mr. and Mrs. Smith". "The Reverend" is never used in direct address. If addressing an envelope (and therefore third person) it should be "Mr John and The Reverend Jane Smith".
I would say that Doctor is the most appropriate because it addresses her status as a physician.
The one mailing the other is more dominate
You may address them as the Drs. Younger. If Dr Layne is female , and uses her maiden name professionally, then the address should say Dr Cole Younger and Dr Layne Holland. If they are married and hyphenated, the address will be the Drs Cole & Layne Holland-Younger.
The same way you would start a letter to a male pharmacist, probably "Dear Doctor X"
A female doctor in Spanish is Doctora