Both are Ethernet compatible.
Supposed to share GPS data and sonar IF the 1197 has the current software version. But some are not having any success doing it. Some even state they don't like the 2D sonar performance of the 1197 with current software.
Software version 6.640 or higher has to be on both units for them to "talk". The XX9 models started with a version higher than 7.000 so that makes it mandatory for the 1197 to have at least 6.640
Ethernet is the only connection you can use. Ethernet does not write the waypoints to the other unit's memory. It displays the other units waypoints in a different color.
The issue with sharing a transducer, is the one at the bow will be getting sonar data from the transom. Not very effective when using some presentations (vertical).
While you can adapt the SI transducer that comes with the 899 for trolling motor mounting, if you have a variable speed trolling motor, the likelihood of RFI is high. There is a trolling motor puck version of that transducer that has extra internal shielding (XTM 9 SI 180 T), or the same one you have with the 1197 (XHS 9 HDSI 180 T) that is more suitable for that application- just need a trolling motor mount for the HDSI.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1