First and second door leaves are pivotable with respect to a door frame and each door leaf has a hinge edge at which it is hingedly attached to the door frame. Each door leaf is provided with a door closer and a pull arm for closing the door leaf. The door frame is provided with a guide rail mounted to the door frame for guiding movement of distal ends of the pull arms longitudinally of the guide member. During closing movement of the second door leaf, the distal end of the second pull arm moves along the guide rail initially in an allowing direction, away from the hinge edge of the second door leaf, and subsequently in a blocking direction, towards the hinge edge of the second door leaf. During closing movement of the second door leaf, a blocking device allows movement of the distal end of the second pull arm past the blocking device in the allowing direction and selectively prevents return movement of the distal end of the second pull arm past the blocking device in the blocking direction.