this is good information. I may have to help Larry figure this out though. I would just recommend getting a depth finder that has gps built into it. Then linking two different mounted transducers to the same gps/depth sounder unit. Or getting two units that combine gps/depth sounders.
I think that the unit you are referring to uses the standard NEMA sentences to talk from one unit to the other.
I know that Pat Hahs of Kentucky Hydrografx uses this type NEMA stuff.
They make some data loggers that will do this also.
Regards,
Moose1am