Change channel in Airport Express when extending network
You've identified the big problem with WiFi backhaul. You have to have both radios on the same channel and half the time the radios are sending traffic between each other and the other half talking to clients.

Since wifi is half duplex and everyone has to wait for the last person to finish talking before the next can speak - you run into serious limitation once you're at the 10 to 30 client number.

For almost all cases where you want channel diversity - run a wire so that your backhaul is over power line / ethernet / coax cable - anything but WiFi will make your network so much faster and easier to manage.

For corporate settings, you almost need a third radio dedicated to the backhaul to make most mesh networks work best or bite the bullet and run wired backhaul there as well. Normally power over ethernet means wired backhaul is doable for large deployments as well as for small home / office situations.