Hello,
I have installed two Lowrance
GPS / Sonar units, both with the transducer mounted inside the boat. The first transducer was epoxied to the hull, the second is held wit toilet bowl wax. Both work great and I haven't been able to find any loss of signal. I don't use mine for fishing, only to see the bottom.
Note that if you have two transducers on the same frequency, the displays of both may show 'weird' results. My boat has an old Datamarine depthfinder that works fine. If my Lowrance SONAR is set to dual frequency (50 and 200 hz) the Datamarine will show the current depth, then a real shallow number, then a real deep number, then the current depth. When I set the Lowrance to 200 hz, the Datamarine works normally.
Good luck,
Barry