Another Elite-7 HDI will work and Lowrance makes a fairly inexpensive bracket to mount the HDMI transducer on the TM...around $20. They can be connected thru the NMEA ports on the back for waypoint sharing. Also understand NMEA2000 and its capabilities. Waypoints are transmitted upon entry from one unit to another. They are not shared per se. Lets say you have two units linked through NMEA2000 and one of them is off and you enter a bunch of waypoints on the one that is on. When you turn the second unit on it will not show these waypoints. You can transfer them by downloading them on a chip and then uploading them from the chip onto the second unit.