"You stated that the adapter should be given a channel number "that's as far away as possible from that of the main wi-fi router"."
That's the wrong advice for wireless extenders. The channel number should always be the same as the router's. So, your setup is working as it should.
"The only slight disadvantage I can see is that once a device has latched onto one of the signals, it stays connected to the original source even if moved close to the alternative."
When the signal becomes too weak it's unusable then the wireless client should then latch on to the stronger signal. Can you test this? Are you having wireless connection problems?