Not sure why you were running the XTM-9-20-T on the 1199 at the bow since that would reduce it to a 2D only unit.
I am editing my post because you indicated you have 2 DI units, but I now realize you have 2 SI units. Your best bet is to mount your SI transducer on the trolling motor.....You will not need a Y cable or US2 built into the trolling motor since the SI transducer has the 2D already built in.
That transducer cable will fit the slot of the Terrova. If you do a search, there are online videos showing how to install the transducer cable in the slot.