The connectors on the back are color coded. To link them, you need to put an Ethernet cable between them. This would be on the golden yellow colored connector. There are two on your units I believe. This is so you can daisy-chain a number of units together. With 2 units, a single Ethernet cable can be connected to either of those on each unit. I believe the shortest Ethernet cable is 6 feet. Once you do that, they can share a map chip you put in one unit, and waypoints will be shared automatically. Set a waypoint on one unit, and it will appear shortly on the other. Note that with this feature, if you want a waypoint to go away forever, both units must be on when you delete it.