I have a layout with two separate tracks -- one running conventional trains and one running TMCC. I have one TPC 400 with two power units. Can one TPC 400 be wired to power both tracks, or do I need a separate TPC 400 for each track?
If you want separate power control of each track you would need one TPC for each track. If you a running one track strictly TMCC wire that track straight 18 volts and the other to the TPC.
If you want the option to run either track as either conventional or TMCC/Legacy, then you might consider adding a Legacy Powermaster instead of an extra TPC. They are a lot less expensive than a TPC unit, and will give you up to 10A of fully controllable power. Plus they are simpler to install, and have much better short circuit protection than the old TPC units do.