When your letter is to more than one person, write out all of their names separately, separating them with commas. For example, "Dear Mr. Hobbes, Ms. Luxe, and Mr. Hopman." For married couples, if one person in the couple has changed his or her name, you only need to use the last name once. For instance, "Dear Mr. and Mrs. Smith."
In the end, this is all about respect for your recipient. If you’re offering a real solution to a real problem, people want to know about it. By following proper and well_understood formats and keeping your writing clear and engaging, you’re making it easier for them to solve their problem. Provided that you’re honest and have the skills and talents to back up your claims, writing a good, solid letter makes it more likely that both you and your reader will be satisfied. Try it the next time you have to write an important letter.