Real life track curcuit uses either voltage or audio frequencies. If it's voltage, you can't overlap them as circuit is either opened (electricty flows) or closed (electricty doesn't flow).
If it's AF, it can use multiple frequencies and each one represents different circuit, but you still wouldn't like to have them overalped. AF is used because it's the only way to have track circuit on the tracks which are continuoulsy welded and this is today's standard in Europe. You can't have insulated sections on continuously welded tracks.
If you need to cover a track and use it as 2 circuits for passanger train and 1 as cargo (they are longer), you just program the signaling system in such way that it can enable so-called short entry into the station (where you occupy just 1 circuit and the other one can be used by other tran) and long entry where both are occupied.
Good question is how to do this in Trainz. I think EIT path's can't overlap so you can't have path A-B-C which would be short entry and A-B-C-D which would be long entry.