I used to just add the guy to his girl bff's home before they started to romance each other and theystarted living together as roommantes before becoming a couple though I had to get a marriage last name chooser because what I really want is the choice of last name taken when they get married as the guy sometomes is an only chile or only had a brother and the girl had two sisters.They'll rake n his family name in those cases though they'll take her last name if he's got two sisters to continue his family name.
I'd advise just loading the household of the sim to be moved out of their home and have them get packed up to move then evict them if they live alone.They can be joiniing the other sim's household before getting married if they're already bff's or best friends and romance is about to start though some interactions would usually wait for after marriage.I would have the sim moving in get a few things packed if they don't live alone like they live with theri parents and they can find own place and move out that way if they're the only one leaving.