Proper etiquette, by formal traditional standards, is that every person should be addressed by the name and title that he or she prefers. This is the kind of basic interpersonal knowledge that a hostess should bother to find out, and keep track of in her contacts list, or visiting book, or wherever she is storing her address list (and waiting-bee or long-leadtime brides should start keeping track of proactively.) This includes both the old-fashioned standard of women styling themselves as “Mrs John Hislastname”, or the equally oldfashioned standard of married women styling themselves as “Miss Ann Maidenname”, or the new standard of women who kept their own name in whole or in part using “Mrs” regardless to show that they are married. Any of these is correct if that is what the lady prefers. So if you know your guests preferences, your problem is solved.
On social correspondence, first names and surnames are not used together except where necessary to distinguish an ambiguity. Hence on an invitation or inner envelope you would use “Ann and John” for informal correspondence or “Mr and Ms Smith” for formal correspondence. Their sons would be Mr Thomas, Mr Richard, and Mr Henry (so as not to confuse them with their Da,) and their daughters would be Miss Smith, Miss Caroline and Miss Dianne. So if your concern is only for placecards, inner envelopes, the invitation proper and such things, your problem is solved even if you don’t know the guests’ preferences.
It is on business documents that title, given name and surname are used together. The outer envelope is a business document between the sender and the postal service provider. In English-speaking countries outside of the USA, the norm is to address the outer envelope to the ONE person in the household, typically the wife, who is responsible for the family social calendar. If she happens to go by her husband’s name, it should then be addressed to “Mrs John Smith”. If she goes by her own name, it should be addressed to “Ms Ann Smith”. So as long as you are not in the states, your problem is still resolved.
If you are trying to address an outer envelope in the United States, and don’t know the guests’ preferences, you need to resort to community norms. Modern protocol guides recommend using “Ms” with the lady’s own forename, keeping the title, forename and surname for each party together, joining the two names of a married couple with the word “and”; and sending separate invitations to room-mates, adult children, and adult siblings. Protocol guides disagree on the order of names: the traditional standard is that the man goes first any time that by so doing he can make the lady safer or more comfortable: when crossing a minefield, for example, or finding a way through a crowded pub where there is no hostess to lead the way, or when facing the rigours of a transcontinental journey together on the back of an envelope. The modern standard is that such sexist considerations are demeaning and the names should be in alphabetical order. The notion that “ladies first” is traditionally correct on outer envelopes and formal correspondence is simply mistaken, and the notion that “the person you are closer to” should be named first is unkind — you never want to imply that you play favourites between two related guests.