They will share everything but the chip information. All you should need to newwork the two is an ethernet adapter for each unit and a cable. If you add anything else such as another sonar, 360, i-pilot link, then you'll need the 5 input ethernet box.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner