As I recall, when addressing multiple gentlemen in a letter, the form went something like this:
Messrs John A. Anderson and Stephen B. Black
(If the above is not totally accurate, please correct it.)
What would the corresponding form be for addressing multiple ladies in a letter, (assuming that one would use the plural of Ms)?
The feminine equivalent for Messrs [Mes Sieurs} is Mesdames, which is practically completely obsolete.
I would not worry too much about this, but use the Ms. and follow it with the names as necessary.