Improving Signal:Noise of CW signals by 3dB

There is a very interesting article in this month's CQ Magazine by Pete N8PR. He notes (pun intended) that he is able to discern an improvement in copying weak signals on CW by listening to a station with both of his receiver's VFOs tuned to receive the station but with a separation between them of 65-80 Hz. The article goes into the details of how and why this is theoretically possible,

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