What your talking about is called "Detector Cross Talk" this becomes more of an issue when the detectors are near the same operating frequency. My two brothers above both have mention hunting with each other while one was using a 1270 and the other something else, but SL52's F5 and my Omega would weird completely out when we got within 20 feet of each other. but it wasn't a noise problem, they just sort of didn't work (cancel each other out), he would have a good signal and call me over to check it and I'd say there isn't anything there at all! then he would put his F5 back over the target and get the same results I did, nothing! I have had other encounters with detector cross talk where it was noise.
So, detector cross talk don't always act as one would expect.
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Mark