Wavelength selective switch (WSS) is the crucial component in the reconfigurable optical add/drop multiplexer (ROADM), which plays a pivotal role in the next-generation all-optical networks. We present a compact architecture of twin 1×40 liquid crystal on silicon (LCoS)-based WSS, which can be regarded as a 4f system in the wavelength direction and a 2f system in the switching direction. It is designed with theoretical analysis and simulation investigation. Polarization multiplexing is employed for two sources of twin WSS by polarization conversion before the common optical path. The WSS system attains a coupling efficacy exceeding 96% for 90% of the ports through simulation optimization. The 3 dB bandwidth can be achieved by more than 44 GHz at a 50 GHz grid for all 120 channels at all deflection ports. This work establishes a solid foundation for developing high-performance WSS with larger port counts.