It requires you to use this switch http://www.humminbird.com/Products/AS-ETH-5PXG/ to link to other units per the 798 details on Humminbird's site http://www.humminbird.com/Products/798ci-hd-si-Combo/ but I've never tried linking two head units so I don't know what functionality it gives you.

It is an SI unit, but it can also handle 360 imaging if you have the accessories for it , as well as i-Pilot (Helix 5 SI units cannot use these accessories) and Humminbird's autochart map-making software.

My unit has the most up to date software version installed on it.