A refrigerator appliance includes a cabinet, a first door, a second door, a mullion, and a lock. The cabinet defines an internal cavity. The first and second doors are configured to transition between open and closed positions. The mullion is secured to an edge of the first door and is configured to transition between active and inactive positions. In the active position the mullion is configured to span a gap between the first and second doors. In the inactive position the mullion is retracted away from an edge of the first door. The lock is configured to, in response to the first door transitioning to the respective open position, activate and lock the mullion in the inactive position. The lock is configured to, in response to the first door transitioning to the respective closed position, deactivate and unlock the mullion such that mullion transitions to the active position.