WiFi Channelization for 40, 80 and 160 MHz
Your receiver does not know what BW you are using. Hence it will always be tuned to receive 20MHz BW (ch 36 or 40). So for the preamble part you need to transmit in channel bonding (2 x 20MHz) mode. Once the receiver has detected preamble it decodes the header to find out that data part is 40MHz. You need to turn on a bit in header for this (forgot the exact bit number) you can find it in standard. Same case for 80MHz BW. Preamble part is 4 x 20MHz.
Yes you need to continuously tune to 20MHz channels in the receiver. Otherwise no way of knowing what BW transmitter is sending. For fast synchronisation this is the feasible method. Once you detect preamble you need check for that header bit in both primary and secondary channels to check for 40MHz BW.