You will need the AS ETH 5XPG ...
https://www.humminbird.com/productDe...id=728&id=1817
You will need an Ethernet cable long enough to reach from each unit to the AS ETH 5XPG ...
https://www.humminbird.com/productCa...74&facets=7319,
You will need an Ethernet adapter cable for each HB unit ...
https://www.humminbird.com/productDe...eid=728&id=868
The ipilot should have come with an Ethernet cable - it will also connect to the AS ETH 5XPG ...
The AS ETH 5XPG requires 12v power so plan it's installation before choosing Ethernet cable lengths ...
The 999 and HELIX 7 CHIRP G2N will share waypoints, accessories, and Sonar (CHIRP will have to be shut off the HELIX to share Sonar) ...
Rickie