You are picking up the local oscillator from the "other" radio.
When it gets on a harmonic of or the desired frequency of the "first", radio it is strong enough to block out signals.
For example, most AM receivers use an intermediate frequency of 455 khz.
If you tune to AM 1000, the local oscillator of that radio will be 455 khz higher, at 1455 khz.
When it mixes that local oscillator sgnal with the incoming, desired signal from AM 1000, one of the products is a 455 khz
signal , (because that's a numeric difference between the two signals). This is what then goes through the
intermediate frequency amplifiers and later gets rectified to hear the audio.
Depending on the design and shielding of the radio, it is also radiating a signal at 1455 hz, which would interfere with
reception of AM 1450 and 1460. You might hear a 5000 hertz squeal on those two frequencies if there are signals to hear
there, or you may just hear "strong quieting", which you demonstrated in the AM video.
Not all radios have the same IF scheme, or the same amount of oscillator leakage, which is why it may not seem
to work the same way with two different radios.
