899ci (internal GPS) and 898c can share waypoints, accessories, and all Sonar ...

898 can use the external GPS either connected to itself or thru Ethernet if connected to the 899 ...
898 can also use the internal GPS in the 899 ...

Keep in mind that the chosen GPS antenna (in each unit's Ethernet Menu) should be as close as possible to the xducer being used for that unit for best waypoint accuracy ...

Sonar can be shared from either unit thru Ethernet ...

You will need the AS EC QDE for the 899 ...
https://www.humminbird.com/productDe...eid=728&id=868

And an Ethernet cable long enough to reach ...
https://www.humminbird.com/productCa...74&facets=7319,

Rickie