What is the correct salutation of a letter to two women with same last name?

This could vary depending on the ladies' marital status. If one is married and the other a single (never married) lady, you would write: "Dear Mrs. Jones and Miss Jones. If both are married or have been married, it would be: "Dear Mrs. Joe Jones and Mrs. Jane Smith. If both have never been married it would be Dear Miss Jones and Miss Smith. If you don't know, you could always inquire of the ladies how they would prefer to be addressed.