You can use any combination of power on the input side of the TIU, the only rules are, you can't connect TIU outputs together. So, you could power the TIU with one, two, three, or four transformer outputs, totally your choice.
I actually have the variable channels on my mainline and sidings, that allows me to do conventional testing when necessary. With the TIU variable channels configured in FIXED mode, it's pretty flexible. Full voltage until you select a track and vary it's voltage, then it drops into variable mode automatically.