I have two Raymarine depth sounders, same frequency, one each in the bows of
my catamaran, and they don't interfere at any depth.
Previously, I had two Datamarines, same frequency, in our old cat, no
interference.
Technicians with the mfgs suggested there would be the problem you mention,
but I found it not to be the case.

"George Pinson" wrote in message
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As far as interference with each other is concerned, it
depends on the depth. If the two tranducers are the same
frequency, and you reach a depth at which the cones overlap,
they WILL interfere with each other.

As noted, a switch box will allow you to use one tranducer
with both units, or mount both transducers, and only have
one on at a time.