When sending a letter to someone with a Ph.D., the envelope should be addressed using the title, "Dr." For example, "Dr. John Doe" should be written above the address. With a Ph.D., the designation is primarily used professionally, but it is not bad etiquette to use the title socially.
When addressing a letter to a married couple with the same last name, and one of the pair has a Ph.D., the two are addressed as Dr. and Mrs. John Doe or Dr. Jane Doe and Mr. John Doe. If both have doctorates, they should be identified as The Doctors Doe, Drs. Jane and Joe Doe, or Dr. Jane Doe and Dr. John Doe. If the couple do not share the same last name, they are listed separately: Dr. Angela Williams and Dr. John Doe. In the case of double doctorates, the order of the names does not matter.