The issue is the heat dissipated in the wire and it doesn't really matter which circuit they are on.
If "X" amount of current goes down one wire and comes back on another in the same pipe, the accumulated heat is double.
I have heard convincing arguments that when you have several conductors on the same circuit going one way and sharing the available current since they are paralleled you should not need to derate beyond one conductor for each circuit but the language is not there to allow it.
An example would be 10 100w can lights on the same 20a circuit but separate switches on the near end, going through a raceway. As a sanity check, you could only have one neutral coming back but each hot needs to be counted at 20a because that is the size of the O/C device. (11 current carrying conductors).