This paper proposes sensorless control of the entire speed region for a single inverter dual parallel (SIDP) interior permanent magnet synchronous motor (IPMSM) drive system. An active damping control method that adjusts the secondary motor torque to maintain synchronization with the main motor is adopted to address the instability of the SIDP IPMSM drive system. A sensorless control method using the extended back-electromotive force (EMF) is utilized for the SIDP IPMSM drive system in the middle- and high-speed regions. An I-F control method is employed to enhance stability during low-speed operation. Additionally, a flux-weakening control strategy is implemented to extend the operating speed range into the high-speed region. The effectiveness of the proposed sensorless and flux-weakening control methods for the SIDP IPMSM drive system are validated through experimental results.