Ok, definitely not differential drive, so it’s not that. I’ll apologise in advance for asking, but the squelch isn’t turned up, is it? Failing that, the next steps I’d take (and it all gets a bit painful from here) is to
buzz not just that the right things are connected, but also that nothing wrong is connected (ie no shorts to other signals). That takes a while because a) there a lot of possible combinations, and b) because you aren’t expecting to get a buzz, you have to be really careful that you’re actually making connection with your tester to the signals that you’re buzzing.
take the switch out of the circuit again - ie take it back to exactly as per the schematic with only one radio and see what happens then.
If none of that shows up anything, then we’re into poltergeist territory… 😳