The correct way to address an envelope to a married couple that have different last names is to use both of their names. For example, you could write Mr. Johnson and Miss or Ms. Dwyer.
You can address it to only one, or address it to both using both names. Either way is acceptable.
* Senor & Senora ____________________. Senorita is single, Senora is married.
Dear Mr. (name) and Mrs (name)
No, do not sent separate thank you notes and address the envelope and put both names of the married couple on the top of the card to thank them for the volunteering they did.
The envelope should read: Mr. John Doe & Dr. Jane Doe. If you don't know her first name then put: Mr. & Dr. John Doe.
Senior Doctor first if it is a medical reason or unmarried, Man first if it is a married couple.
On the outer envelope it should read: PERSONAL Mr. & Mrs. John Doe c/o Zero Acme Co., 0000 Hawthorne St. San Francisco, CA ZIP CODE #
You may say to Signore & Signora or abbreviate it as Sr. & Sra.
* There is absolutely nothing wrong with it and it's socially acceptable. It would be better if the females return address was on the envelope (to her friends) and put his return address to his friends. As far as married couples you may know put your own address on it and not your fiances.
"Mr. & Mrs. C. Smith" if you know his first initial. If not, then "Mr. & Mrs. Smith" would be correct.
I was always told that you never seperate a man's name. So, if you are addressing a card or envelope informally, and the couple is married, it should be addressed: Jane and John Doe