You bet there is another cord needed. It is generally going to be called the NMEA 2000 Starter Kit, which has several cords, T's and a power lead for the network.
What is this network..? ? It will connect both of your Carbons so that either Carbon can use ALL of your devices.
In other words your front Carbon will be able to use it's sonar, AND THE REAR SONAR. ... Once you decide where you want to put the various Sonar devices you can use them on either device.
Since the 3D structure scan will build a model of your bottom after multiple passes, there is no reason why it couldn't be mounted from or rear. Check out the spotlight scan for the front trolling motor.