This study employs topology optimization to reduce the mass of motors. Minimizing the mass can decrease the required manufacturing resources and lower energy consumption during operation. This approach supports global environmental protection demands, particularly energy usage and metal resource conservation. Previous research has primarily focused on optimizing the cross-sectional shapes of the stator or rotor separately. However, it is essential to note that the optimum design for the stator or rotor is only ideal when considering the corresponding given rotor or stator conditions. Therefore, this study aims to optimize stator and rotor designs simultaneously, providing a more comprehensive solution. The proposed optimization method utilizes multi-material topology optimization, employing two NGnets to define the design values for the cross-sectional shapes. The results demonstrate a reduction in motor mass by nearly 70% compared to the base model while maintaining the same average torque. Furthermore, the simultaneous optimization yields a structure lighter than the combined structure created from the individual topology optimizations of the lightest rotor and stator shapes.