If you are only going to use 1 unit at a time ... There is the HB US3 switch which would allow each head unit to be able to use the rear xducer/xducers separately ...(GPS, Nav data, Temp would come from their own respective head unit) ...
http://www.humminbird.com/Products/US3/-----
-----
Ethernet can be run from your 998c to a bow mounted "Ethernet capable" HB unit ...
Depending on which unit you desire for the bow mounted unit will determine which of the following they will share thru the Ethernet ...Sonar, GPS, Nav Data, Temp ... (No map sharing) ...
If your desired bow unit choice is also a HB SI unit ... They can share 2d, DI, SI, GPS, Nav data, Temp thru the Ethernet ... (No map sharing) ...
NOTE ...: Onix and Ion will not Ethernet to any HB unit except Onix to Onix and Ion to Ion ...
If you can tell us which unit you are interested in ... We can tell you which of the features could be shared thru Ethernet ...
Rickie