Part of the agreement to let all states have their independence would be some basic requirements. One of those requirements would likely be that the new states have to allow freedom of movement and migration from the other US states, at the very least for several years after the national divorce. Ideally the president orchestrating this would negotiate some sort of basic EU-style customs and trade union. So the states can set their own laws and national policies, but people can move freely between the various new nations. Hell, most states would likely want this anyway. Does Mississippi really want to trap blue voters in its borders without giving them any relief valve? I would think they would be far happier just maintaining open border deals with the other new states, thus allowing people to simply vote with their feet if they don’t want to stay in Mississippi. The alternative is that you end up having a mini civil war inside the borders of just one state.