You address an envelope to two doctors as: Drs. John and Jane Doe or, Dr. John Doe & Dr. Jane Doe. If it's to two doctors in the same office then you would address it to: Dr. John Doe and Dr. John Smith.
If they are sharing accommodations: Ms. J D Jones Ms. A L Smith Listed alphabetically on separate lines. If they are in a committed relationship (Lesbian partnership/marriage) Ms. J D Jones & Ms....
With the new generation 'Ms.' is used often if you do not know whether the person is single or married. The correct opening of the letter should be: Example: Ms. Jane Doe and Ms. Joan Smith. If you...