Locally to me, I’ve split National Trails into parts that allow the bits that are also part of other trails to be added to them. If, separately to that, there is some “official division” of the trails it’s also OK to do that - you can have “day 2 part 1” and “day 2 part 2” or similar.
They don’t show up well within iD, but many LDPs have several levels of superrelation / superroute.