In the Internet age when sending breezy emails is de rigueur, a business letter stands apart for its thoughtfulness, planning and care. A thank-you letter is no exception. During the course of business correspondence, you may be called upon to send a thank-you letter to more than one person. Letter-writing etiquette can be strict or loose in this regard, based on the relationship of the recipients to each other, as well as to you.
Adapt a thank-you letter to the man and woman based on your level of familiarity with the recipients. If you know the couple or are on casual terms with them, address them on the envelope as "Jane and John Doe," and say "Dear Jane and John" in the letter's salutation. If you don't know them or believe them to prefer traditional forms of address, use "Mr. and Mrs. John Doe" on the envelope and "Dear Mr. and Ms./Mrs. Doe" as your greeting.
Address a married couple in a thank-you letter based on a number of variables. If the woman prefers "Ms." use "Mr. John Doe and Ms. Jane Doe." In business correspondence, "Ms." should be standard, unless other preferences are known. If she uses her maiden name, use "Mr. John Doe and Ms. Jane Kennedy." For business correspondence, use either the woman's name or the man's name first; gender does not matter.
Make adjustments based on the title or status of the individuals. If one party among the pair outranks the other in terms of educational or professional designations, elected office or military rank, list that person first, for example, "The Honorable Jane Kennedy and Mr. John Doe." If they are both doctors and use the same last name, use "The Doctors Doe." If they are both doctors and have different surnames, state either doctor first: "Dr. John Doe and Dr. Jane Kennedy."
Address unmarried couples as "Mr. John Doe and Ms. Jane Kennedy." In the case of a man and woman who are not a couple but rather business associates, both of whom you want to thank, send separate thank-you letters addressing each individually. In the letter, use the recipient's first name if you feel comfortable doing so; otherwise, use Mr. or Ms. Doe.
Occasionally, a thank-you letter isn't strictly a business letter or a social one. Letters that veer on the social side would privilege the female over the male in the sequence -- "Mrs. Marcy Townsend and Mr. Frederick Bowles" -- whereas a business-oriented letter would favor alphabetization and reverse the order.