Without specific info regarding the models being referred to - difficult to answer the question.
You mention "combination chart plotter/radars" and "chart plotter/ fish finder"
Need to know is the Radar a completely separate unit NOT connected to the combination Chart plotter/fish finder ? How many units are there ?
as you indicate you need answers ASAP - Suggest you contact Marcom Watson, who are near to you :-
MARINE ELECTRONICS AND COMMUNICATIONS
Speak to Colin Kew
Telephone Number Local: 03 6331 2711 International: +61 3 6331 2711