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."
Then you arouse their desire by explaining how you can do this — for instance, by showing how successful you’ve been in other positions, or by explaining the specialized training you have relevant to the job. Avoid jargon here — explain clearly and concisely the benefits of the solution you offer. When I worked in marketing, I learned an important lesson: nobody buys anything based on features (and “buy” is used in its loosest possible sense, like you “buy” an idea). You have to explain exactly how those features will benefit the potential buyer.