The sender can address two married doctors as "Doctors" or "Drs." followed by the first names of each person and then the last name. The first names may also be omitted if desired and "The Doctors" followed by the last name used. Another way to address the two doctors would be to simply include "Doctor" or Dr." followed by the full name of the first person and then the same for the second person.
For example, if Jane Doe and John Doe were both doctors and married to each other, they could be addressed as "Doctors Jane and John Doe," "The Doctors Doe" or "Dr. Jane Doe and Dr. John Doe." The husband and wife's names can be reversed if desired.