What is the proper salutation for a letter to two females?

Two acceptable abbreviations for the plural of Ms. are Mses. or Mss. If you aren't comfortable with that format or the two females have different last names, you may address them individually:

Ms. Mary Smith and Ms. Jane Jones; Dear Ms. Smith and Ms. Jones,

Mss. Mary Smith & Jane Jones; Dear Mary Smith & Jane Jones,

Prof. Mary Smith and Dr. Jane Jones; Dear Prof. Smith and Dr. Jones,

Mses. Mary & Jane Smith; Dear Mary & Jane Smith,

Mary Smith and Jane Jones; Dear Ms. Smith and Ms. Jones,