This situation occurs among several of our family and friends. With family I address envelopes using both their surnames. But in the case of friends I often don't know the partner's surname, only ever having been introduced on first name terms, so I address it to the partner whose surname I do know. It seems to work, no-one has ever commented or been upset by it.
I'm probably a bit old fashioned about address etiquette. I would never address anything with a full Christian name, not even to a child, I'd put Miss A Smith or Master B Jones. It's the way I was brought up and also taught at school. Our GC love it and say they immediately know it's from me! I told the boys that once they are 18 the 'Master' will become 'Mr' as they are then adults, and they like the significance of that.
I have a relation with a title and an impressive string of qualifications which he always includes on his correspondence. I think it's great that he does that, he worked hard to achieve them. The only problem is that sometimes card envelopes are too small to fit it all on!