The IEEE 802.11 specification for Wi-Fi supports 20 MHz channels utilizing bandwidths up to 160 MHz. However, the support of devices simultaneously transmitting over different, non-overlapping channels, is not specifically addressed. Recently, there has been interest in developing an 802.11 specification for next generation Wi-Fi which has led to the requirement for solutions with efficient frequency resource utilization that are not currently supported. In this paper we continue the study of the Multi-User Parallel Channel Access (MU/PCA) scheme that enables simultaneous transmissions to multiple devices of various bandwidths, while maintaining backward compatibility with 802.11ac. We provide detailed physical layer design methods for MU/PCA, and propose several multi-user sub-channel selection algorithms. Finally we present numerical results which demonstrate the potential performance of the proposed MU/PCA algorithms.