I'll talk about this more once the MVP is set up for Act 2; however, as a sort of overview of how Act 2 is going to be implemented is, after reaching a certain point in the original game (Act 1) there will be a small story beat that introduces Act 2. After that intro plays out Act 2 will start where the party will travel somewhere else that is completely separated from Act 1. After Act 2's short introduction plays out in the new area, there will be an object to interact with if you want to travel back to Act 1 from the new area and vice versa back to Act 2 from the Khrono Manor.
Act 2 is just another Build Update to Mira Co Rescue similar to the previous updates; however, I'm referring to it as Act 2 since it is completely separated from a development standpoint from the original content of Mira Co Rescue (Act 1).