Carolyn wrote to ask for guidance on addressing correspondence to two women who are married to each other and have the same last name. Below are suggestions for a couple I will call Grace and Linda Smythe.
For a formal business letter
On the envelope: Mses. Grace and Linda Smythe
For the greeting: Dear Mses. Smythe:
(Mses. is the correct plural of Ms. Ignore Microsoft's suggestions to change the abbreviation to Mess, Moses, Muses, Messes, Messy, or Miss.)
If you happened to know that the women preferred the title Mrs., you would use these forms:
On the envelope: Mesdames Grace and Linda Smythe
For the greeting: Dear Mesdames Smythe:
(Mesdames is the plural of Mrs.)
For an informal or friendly business letter
On the envelope: Grace and Linda Smythe
For the greeting: Dear Grace and Linda: or Dear Grace and Linda,
Either woman's name can come first.
Whenever you are not sure how someone wants to be addressed (for example, Ms. or Mrs.), feel free to ask. People like to be asked about their preferences.
As for asking me, I am receiving more questions about salutations and other topics than I can answer. That is why I cannot promise to respond to individual questions. If you send me a question that will interest readers here, I will answer it here. For questions about salutations, please see earlier entries such as this one on addressing doctors and others and this one on writing to Jrs. and Srs.