We faced this exact situation on our Pearson 303. The boat has original Datamarine depth and speed gauges and transducers, both of which still work. We added a Lowrance GPS
that came with a fish finder
. The standard transducer that came with the Lowrance was a transom mount like small fishing boats use. I believe the Datamarine depth transducer is mounted just forward of the keel and to starboard. What we did was to take a plastic bag full of water, place it directly opposite the Datamarine transducer (i.e., same location but to port), ran the wire through the cabin to the head unit, then turned it on and checked to see if we got a signal, which we did. This is commonly called a "shoot through the hull" installation. It usually only works if you have a solid glass hull - cored hulls can interfere with the signal due to air voids. We also checked the Datamarine unit, which worked fine. I believe Datamarine used a different frequency than what is common these days, which is probably why the two units don't interfere with one another. Once we had our location chosen and know everything would work, we then mixed up some epoxy
with filler to peanut butter consistency, slathered on the transducer, put some where we were about to install, then worked the transducer to the hull to ensure there was no air between the two. Try the first steps to see if yours will work, then you can decide how you want to mount a unit. Some people use silicone, since it wouldn't need to be ground and chiseled out like epoxy
if you ever want to remove the unit. Others install a piece of pipe filled with oil. You can find lots of options with a web search. Of course, it you want another hole in the boat, then you will need to do a traditional install. Even then, you should be able to see if the units will work together by testing with water before you start drilling. Now that we have both units, I almost always use the Lowrance, as it allows me to see contours and compare them to the chart on the split screen.