I don't believe this is what actually happens with RayMarine chart-
plotters. From what I've seen, it uses a single talker for the input and
when that input fails, it switches over to the other source.

I know that's what it does when dealing with GPS from sources on different
busses. I've got one on NMEA2000 and the other on regular SeaTalk. It uses
the one on NMEA2k first (and doesn't give me a choice otherwise) and will
fall back to the one on SeaTalk.

In the case of multiple compasses, which I have, it averages the reading
between the sources.

Is there a seatalk-only heading sensor? I've got a heading sensor but it's
attached to the autopilot, and thus only works what that's turned on. I
tend not to use the autopilot but would like a header indicator.

-Bill Kearney