Duplex control system and control method therefor

A duplex control system (1) is provided with: two control devices (2), one being operated as a standby system and the other being operated as a common use system; a tracking bus (3) which connects the two control devices (2) to transmit and receive a tracking signal (21); and a control transmission path (4) for transmitting calculation information relating to the two control devices. The control devices (2) each comprise: an own-system state determination unit (11) which transmits an own-system state determination result as the tracking signal (21) to the other system via the tracking bus (3); an other-system signal confirmation unit (13) which receives the tracking signal (21) from the control device (2) of the other system via the tracking bus (3) and performs a state confirmation; and a transmission path switching unit which, when the tracking signal (21) cannot be received via the tracking bus (3), causes the tracking signal (21) to be transmitted to the other-system signal confirmation unit (13) of the o