Yes you can Ethernet connect a 798ci HD Si model unit with a 859ci HD unit. You will need an Ethernet Connection Cable (AS-EC-XXE where the “XX” gets replaced by the cable length. They come in 2, 10, 15, 20 and 30-foot lengths) and two of the AS-EC-QDE Ethernet Adapter Cables.
The units will be able to share GPS Receivers, waypoints and 2D sonar.