I think another "official" difference is the seating: 4 adults in a 2-door sedan vs 2 adults and (sometimes) 1 or 2 children in the coupé (e.g. the coupés named "2+2").
For example the classic VW Beetle and BMW E30 are 2-door sedan, even if the VW has a sloped rear unlike the BMW that has a classic sedan shape but just two doors.