The US3 Unit Switch will allow you to switch one transducer between two units (or unit locations).
The EC-W10 is a ten-foot long transducer extension cable.

As for the running two sonar one on stern and one on bow affecting each other? The answer is probably. If both are using the same frequency there is a chance of sonar cross-talk between them which will change with water depth, bottom composition and a little luck. If it shows up on the Humminbird unit try turning on and up the Noise Filter menu.