Standard etiquette requires that you address each guest as he or she prefers to be addressed. Only if you do not know their preference, do you resort to standard forms. Using standard forms is a guessing game because many people either do not know the standard forms, or choose to flout them on ideological grounds. The basic rules are:
A public servant uses the title that goes with his or her service, and is listed first of a couple.
A medical doctor is considered a public servant; an academic PhD is considered a business title and is expected to go by the ordinary title “Mr”/”Ms”/”Mrs”/”Miss” in social circumstances.
If neither member of the couple is a public servant, then the man goes first.
A man’s given name is kept adjacent to his surname.
A lady uses “Miss” with her own given name and her father’s surname, or “Mrs” with her husband’s given name and surname: for other combinations of names she uses “Ms”
A couple living together may well be married for all anyone else knows. Etiquette does not allow for private details about their legal state or lack thereof to be advertised on envelopes. Therefore they should be treated in exactly the same way as a married couple with different names.
Use given names with titles and surnames on business documents or where needed to disambiguate the name; for social purposes use only the titlle and surname (or title and given name, for younger siblings)
So, on the outer envelope (a business document) in the United States only:
- Doctor Susan Smith and Mr. John Smith
- Mr. Thomas Jones and Ms. Mary Doe
On the outer envelope in the rest of the English-speaking world (social envelopes are addressed to the lady of the couple only):
- Doctor Susan Smith
- Ms Mary Doe
In the inner envelope or onn the invitation proper:
- Doctor and Mr. Smith
- Mr Jones and Ms Doe
“Mrs”, “Ms” and “Miss” are fraught with political connotations and mean different things to different age demographics:
For the over-sixty crowd, use “Miss” for a woman who goes by her birth name (both first and last), use “Mrs” for a woman who uses her husband’s name (both first and last) and “Ms” for a known feminist or a woman who insists on her own given name with her husband’s surname.
For the forty-to-sixty crowd, use “Miss” with her birth names for a woman who has never been married, “Mrs” with her own given name and her husband’s surname for a married woman, and “Ms” for a woman whose marital status you don’t know or who has married and kept her own name.
For the under-forty crowd, use “Miss” for girls who are not yet adults, “Ms” for unmarried women or women whose marital status you do not know, and “Mrs” for married women.
The rule that academic PhDs, not being public servants, do not use their titles is also contentious. Many PhDs are of the opinion that having worked just as hard for their doctorate as a medical doctor has, that they are just as entitled to use the title Dr socially. Presumably they can be trusted to say “sorry, my degree is in medeaval literature” if someone at their table has a heart attack and the other diners shout “Doctor — save him!”
Obviously it is far, far better to find out individual preferences.