firstly with what you’ve described with 2 Z-4000’s aka 4 channels of power you’ll want to take each Z-4000 output to a different TIU channel. As for the variable channels you can choose to use those to where their connected to control voltage or if you use the Z-4000 handles simply set the variable channels to fixed witch as it implies makes them operate like the fixed channels and allows power to flow through them with no restrictions. You also will want to power the TIU with a external power source so that it always has power otherwise fixed 1 must always be powered for any other channel to work.
another thing you could do so that current doesn’t have to flow through the TIU is to wire it in passive mode to add the DCS signal on the tracks but like I said doesn’t run the current through the TIU.
