Welcome to the Humminbird Side Imaging Forums Charlie1.
Trout Bum is correct. You will need two (2) of the AS-EC-QDE Ethernet Adapter cables plus an AS-EC-XXE cable of the correct length. That AS-EC-XXE cable comes in 2, 10, 15, 20 and 30-foot lengths and the length of the cable replaces the “XX” in the part name. So a 10-foot cable would be AS-EC-10E.
As far as sharing transducers goes; you will need to make sure that both units are running the correct software version (6.940 or higher). You can check your “My Humminbird” account to see if it is available yet.
What model 859 do you have:
859ci HD
859ci HD DI
859ci HD XD