Ethernet will display the waypoints of one unit on the other in a different color. It does not write the data to the other unit;s memory. Default setting is Share, so you don't need to do any settings.
Those units will not share mapping with any connection. If you want additional mapping other than the included ContourXD on both you need two map cards. They can be Lakemaster or Navionics or a combination of the two.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1