The 859 or HELIX 9 will share Waypoints, Sonar, and Accessories with your 899 ...
You would select which xducer to see in the Networking tab (which will be available when you Ethernet the units together) ..
Waypoints reside in the unit memory that marked the Waypoint ... but can be "displayed" by the other unit thru Ethernet ...
You will need the AS ETH 5PXG switch to connect 2 units and the ipilot (which makes 3 devices) ..
http://www.humminbird.com/Products/AS-ETH-5PXG/
The AS ETH 5PXG requires 12v power ... so plan its installation place ...
You will also need Ethernet cables long enough from each unit to the AS ETH 5PXG ...(and one Ethernet cable from the ipilot to the AS ETH 5PXG - if you don't have one already for the ipilot) ...
http://www.humminbird.com/Freshwater...s/?facets=7319,
You will also need an Ethernet adapter cable for each unit ... (2 total) ...(you may already have 1 if your 899 is Ethernet connected to the ipilot) ...
http://www.humminbird.com/Products/AS-EC-QDE/
Rickie