I would have two Helix 9 SI GPS units with one transducer on the transom and the other on the trolling motor. That way the sonar data you are viewing will come from where you are on the boat.
Exchange the included transducers for the HDSI so you get sharper images.
Connect them with Ethernet and you can share GPS data, sonar data, and accesories.
You could go with Andrew's suggestion and could view the console Side Imaging data at the bow with a 2D or DI model since those will share sonar technology if you want to save some cost.
I prefer sonar data to originate from my location, not a boat length away.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1