The gist of the answer is to use the traditional French honorific plurals, Mesdames or Messieurs. As the column elaborates:
GENTLE READER: "Mrs. and Mrs.'' not only encounters the problem you mention, but it is jarring to those who know the traditional rule that "Mrs.'' is never used with a lady's first name. Furthermore, those who violate that rule do so to indicate divorce or widowhood, neither of which is appropriate here.And massmarrier adds that these will not be odd plurals for very long.
Miss Manners has a simple solution:
Use the plural form of ``Mrs.'' or, in the case of two gentlemen, the plural form of "Mr.'' These are, respectively, "Mesdames'' and ``Mssrs'' ("The Mesdames Sally and Betty Jones,'' "The Messrs. Trevor and William Cartwright'').
All right, Miss Manners admits that these are odd plurals. But they are at least traditional and dignified.