N2k bus devices does not normally care who is sending data. They can show e.g. speed data even knowing anything about sender device. This of coarse may denpend of MFD, but e.g. my Garmin GMI 20 does not care. It just shows on devices "unknown device"
Two Arduinos communicate also well. There are examples MessageSender and DataDisplay2, which I have used to test different messages. They does not act as like normal N2k bus device. TemperatureMonitor is the most closes of certified device. So by connecting two TemperatureMonitor to the bus, they do all handshaking and sees each other, but since they are just sending data, they does not listen each other messages. If you change mode to N2k_ListenAndNode and enable message forwarding, then they will forward others messages.
I have to do some more tests. If you have some device sending on bus, then everything works fine, when you connect device. If I start due first and then connect my MFD, it start to work right. If I have due and then connect other TemperatureMonitor, it may get totally blocked.
So you could try cnnecting your MFD, depth sounder and due to the bus and then start due.