Manfrotto | Tripod Accessory Arm for Four Heads (Black) | 131DDB
Amazon.com : Manfrotto 131DDB Horizontal 4 Head Accessory Arm - Black (for Mounting 2 Heads Vertically and 2 Heads Horizontally, 3/8" Thread On All Attachments) : Electronics
Mounting the Manfrotto 131DDB arm to a tripod - YouTube