No need to hook the two together unless you want to run TMCC from the DCS remote. I do not connect mine together. Sounds like you have the Legacy already in place.
For basic setup just take the power wire that you run to your track and put it into the DCS TIU ( track interface unit). Any one of the four inputs. Then run wire from corresponding output of TIU to track. That's it. You can run both systems at once.
Basically the Legacy output wire goes to the outside rails of the track. The TIU output goes to the center rail.