There are times when you need to address a letter to more than one person and there are protocols for how to best do it. Addressing a letter itself is simple and identical for all cases; the salutation you use will vary depending on whom you are addressing. Using good letter-writing protocol will help you make a good first impression.
Addressing the Letter
When addressing a letter to two people, you may either place their names side by side on the upper part of the letter, above the salutation, or you may place one above the other. The same would hold true for three persons, but the letter will look awkward if you go beyond that. Consider whether you should send individual letters to each person. If you are addressing a board or other group, you may address the board collectively.
You may either address each individually in the salutation, or as a group, when writing more than one person. For instance, two men may be addressed as Mr. Tyler and Mr. Young, or as Mssrs. Tyler and Young. Similarly, two women may be addressed as Ms. Smith and Ms. Brown, or as Mses. Smith and Brown. For a woman and a man, break out the addresses and write Ms. Brown and Mr. Tyler. The abbreviations Mssrs. and Mses. may not be universally recognised, so while they are perfectly correct, you may want to break out the salutation to be safe. For doctors, you may use Drs. Williams and Ralston, or spell out the word "Doctors" for a more formal style. The salutation Dr. Williams and Dr. Ralston is also acceptable. Avoid using an ampersand in any of these cases, which will come off as very familiar and informal.