An AP transmits a multi-user first downlink transmission to a plurality of client stations via a first WLAN communication channel having a first RF bandwidth. The AP generates a MAC data unit having a trigger frame configured to prompt an uplink multi-user transmission by the plurality of client stations, and transmits a second downlink transmission to the plurality of client stations via a second WLAN communication channel having a second RF bandwidth. The second downlink transmission includes the MAC data unit. The AP receives the uplink multi-user transmission from the plurality of client stations via the second WLAN communication channel, the uplink multi-user transmission including simultaneous transmissions from the plurality of client stations within the second WLAN communication channel that overlap in time with the multi-user first downlink transmission via the first WLAN communication channel.