this is because first time when d=2focus=center of curvature, a light ray strikes mirror1 it goes parallel and after reflecting from mirror2 falls on its focus that is the common focus which is the place of the object and vice versa.

when d=4f=2c the light ray strikes mirror1and forms an inverted image at the common center of curvature (refer to ray diagrams for concave mirrors) and vice versa