Harry and Severus own the two flats on the top floor of a building. Although there are two separate entrances and they appear to be two different flats, the wall that separates them can be dismissed with the right spell (your choice whether they own the flats because they allow them to hide their established relationship or discover it as part of moving towards a relationship). What happens when someone (Harry, Severus, Hermione, Draco, Lucius, whoever) discovers that the two flats are actually one?