Messrs Jeeves and Wooster
More than one woman: Dames
Dames Pepsi and Shirley
A combination of men and women: Mesdames
This may seem like intellectual snobbery and in some ways it is, but a recent search of completed items shows that "sirs" in a listing (not counting "sirs-e" hardware) has as 20% chance of selling (out of 10 items) and "Messrs" in a listing has a 34% chance of selling (out of 140 items).
And anything that heightens the chance of a sale (within reason) is worth trying...
(I'd be a bit worried if you started putting it on stationery)