If you need to address a letter to more than 1 woman, listing each recipient looks most professional. This goes for both formal and informal salutations on paper and in emails. You may or may not find that you need to list each recipient by title and last name or by the less formal first name, depending on how well you know the ladies. Maintain consistency by writing each recipient's salutation in the same fashion with whichever option you choose.
Determine how many ladies you intend to address in the letter. List their names on a separate sheet of paper if you intend to hand write the note, or type their names in a separate document on the computer if using a text document or email. This way, you won't leave any names out of the final copy.
Place the woman's preferred title beside her name. These include "Mrs.," for a married woman, "Ms." for an unmarried woman and "Miss" for a young, unmarried girl. Use "Ms." for any woman whose title you do not know.
Begin the letter with the word "Dear," followed by each lady's name. Separate 2 names with the word "and." Separate 3 or more names with commas. For example, "Dear Mrs. Riley and Ms. Jones," or "Dear Ms. Stapleton, Ms. Marcus and Ms. Howard." You do not need a comma after the second to last name in the list.
If you know each of the ladies on a first-name basis, you may list their first names only instead of using titles and last names.
Don't assume a woman is married if you don't know her well or are unsure of her marital status. If in doubt, use "Ms." for the title.