I wish to install a 2nd depth sounder.

I have a Raytheon ST60 series sounder and a Navman 450 combo paddle-wheel log
and depth. The Raytheon is already in use but I wish to install the Navman depth
transducer as a backup and a check. It also will measure greater depths than the
Raytheon which sometimes comes up with shallow depths in turbulent water - when
the chart says 3,000 metres. I don't want to adjust its sensitivity.

Questions - Will there be interference to either or do both filter out through
being different wave lengths or somesuch?

How far apart do they have to be?

What else haven't I thought of?