I have the same set up. I fish out of a 16' flat bottom. Have 1 HDS that works both rear transducer and trolling motor.
You will need to get these items to make it work.
Transducer for the TM
15' extention cable for connecting the TM transducer about $25
A switch that you plug both transducers into and then to the HDS $125
Mount HDS on a piece of plywood and you are ready to go.

Yes it is a little bit of a hasel but a lot cheaper than buying to HDS units and you get everything the HDS has to offer. Got my stuff from Bass pro. They have it on the shelf.
Am now waiting on my SS 1 to come in and will be uptown.

Hope this helps.