How should I address several people in a cover letter?

If you are writing to several people, you could, in theory, write their names directly next to one another, for example:

Dear Mr Huber, Dear Mr Moser,

If you are addressing more than two people, or if the names are particularly long, it is better to list them one below the other. This format is much clearer, especially if one person has a double-barrelled surname.