The middle diagram shows the correct equivalent configuration.
The diagram on the right assumes that the potential is the same at all points of the interface between the upper and lower dielectrics. If this is true then the interface can indeed be replaced by a conducting plate and the upper dielectric becomes a single capacitor, as suggested in the right-hand diagram.
However, the interface is a dielectric surface not a conducting surface, so it need not be at the same potential at all points adjacent to each of the two lower dielectrics. The lower surface of the upper dielectric can be split into two surfaces which can be at different potentials. This is recognised in the middle diagram.
