Don't know that much but on Wikipedia two door sedan is defined like this
The Society of Automotive Engineers defines such a vehicle as any two-door model with rear accommodation greater than or equal to 33 cubic feet (930 L) in volume (a calculation made by adding the legroom, shoulder room, and headroom). By this standard, the Chevrolet Monte Carlo, Ferrari 612 Scaglietti, and Mercedes-Benz CL-Class coupés are all two-door sedans. Only a few sources, however (including the magazine Car and Driver), use the two-door sedan label in this manner.
In the popular vernacular, a two-door sedan is defined by appearance and not by volume; vehicles with a B-pillar between the front and rear windows are generally called two-door sedans, while hardtops (without the pillar, and often incorporating a sloping backlight) are called coupés.
The Mazda RX-8 meets the volume requirement to be called a sedan, but it has vestigial rear-hinged rear doors, so some call it 2+2-door sedan. Another term for a coupé endowed with rear-hinged doors is a "quad coupé." However, this may simply be vernacular, based on a possible copyright by General Motors, for its Saturn Ion Quad-Coupe.