First, I have been up there (in that passage) the raster and the digital charts are VERY close to the GPS and consistent. It's possible that they havd a bad GPS day, but I doubt it. It is different in some places with shifting shoals, sand/mud, moving markers, etc. No one has said, the discovered a non-charted ledge in this case. (The Bahamas, for instance, are well known to NOT be accurately correlated between lat-long GPS readings.)

Secondly, unless I'm wrong, each of those units functions independently in terms of zoom and ability to update the route, etc. the may share a chart library, independent instrument inputs (speed, depth, wind speed, direction, etc. and GPS position. It's rare to have just a repeater with a system. The one issue is the tuning of the radar. If you have set the distance on one radar at a range on a 1/4 mile and the other at 12 miles, then may be some issues with tuning of the antenna. I haven't noticed this on our Furuno 3d, but others have questioned their other manufacturer's systems.

Maine has a lot of hazards that aren't well marked or are marked only from one side. If you don't have electronic means, then you have to either take careful bearings or avoid these close quarters dangers. It was my fourth year in that area before I tried going through the place where Archangel grounded. I have a bunch of instruments and use them. I guess I'm timid, but I went around the hazards. In this case it would have taken 20 or 30 minutes longer. (But when I use to snow ski, I found that skiing through the trees was fun...but I found a few of them "got in my way" ;^))) )