How would you address two females living at the name house with the same last name (mother and daughter)? We do not know them by their first names. Would it be Dear Messes. Smith?
The salutation for two single women with the same last name at the same address is the plural for Ms., which is Mses. Or Mss. So, Dear Mses. Smith.
However, if one female is very young, such as a teenager or younger, then she could have the salutation Miss. Then, it would be: Dear Ms. Smith and Miss Smith. However, using the plural Mses. or Mss. for both should be fine.