I don't know if anyone's specifically mentioned it here, but Universal did come out and say that you will be able to link your pre-existing band from USH and their app to the USF app for Epic.
My assumption is that they will be able to work at both because they don't store data on the bands themselves, but act as communication between your band and your unique ID on the app. This stands to reason that a different app would be able to enter the same code and have no conflicts in activation and use across either apps/either parks.