calli1:
I think the main reason for two transducers is for high speed boats.
The transom sometimes lifts out of the water so high on these, that the standard
transducer is no longer submerged, and won't work properly.
There is another reason for having two transducers, which I am now contemplating.
You could get a 2-D transducer with more detail and coverage than the standard HB transducer.
An Airmar, for instance.
The side-imaging, down-imaging will still come from the standard HB transducer, while
the 2-D can be from another transducer.
All the units get their depth from the 2-D transducer.