Sigler
Is he going to move the Head unit to the trolling motor and to the console? If so then he will need hook it up as a single unit.
Humminbird has the TS3 transducer switch to go between the two transducers, personally I liked the older toggle switch better, but it is no longer produced.
These are the trolling motor transducers that I could find listed on HB's site :XTM 9 HDSI 180 T, XTM 9 SI 180 T
I am pretty sure there are others.
What you need depends on how it is going to be used.
George