If you want DCS command control of your MTH engines, to me the simplest solution is to just add a TIU to your existing system and run the MTH engines with the DCS handheld, and the Legacy/TMCC engines with the Cab 2 handheld. Yes there is a connecting cable that will allow you to run TMCC engines with the DCS handheld (but not the other way around), but the diagram below will get you started with little effort. Just connect either fixed channel output to the track power wires going to your layout.
This is called a TIU in passive mode since power does not go through the TIU. If your layout is not wired with the "recommended" way for DCS, you may find some signal issues. However, many people have added DCS to a layout without "DCS wiring" and have had no problems. Give it a try.
A TIU wired in passive mode can not control conventional engines. If you have a transformer with a throttle (ZW/Z-4000/Z-1000), you can control conventional by adjusting the throttle.
So, to answer your question you need to buy a TIU/handheld set and some way to power the TIU (remember, power is not going through the TIU when in passive mode). In the diagram, a Z-500 brick is used (easy connection)
