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.
When addressing an envelope to a female pastor and her husband, you would typically write her name first, followed by her title. For example: "Rev. Jane Smith and Mr. John Smith." If you prefer a more formal approach, you can use "The Reverend Jane Smith and Mr. John Smith." Be sure to include their full names and any relevant honorifics, and place their names centered on the envelope.
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
A female doctor in Spanish is Doctora
The same way you would start a letter to a male pharmacist, probably "Dear Doctor X"
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.