A strict immigration policy heavily weighted again liberals and biased towards merit would need to be established. After all, as their country crumbled under their own idiocy, they’d be clawing to get into the one still based on the Constitution and the Founders’ intent.
I actually believe you don’t need tough border control between the two nations. You could even have open borders. The issues that separate us are socialism and public morality. Those are governed by the laws of the land. I have no problem if the socialist shops in my store, or the gay couple eats in my restaurant. At the end of the day they will receive no entitlement or benefit from the government of my state. My tax dollars will not be “spread around” to them. If their behavior is criminal under the laws of my state they will be punished according to the law. Likewise when I visit the blue nation I will be aware that I can be imprisoned for my speech, that I cannot practice my religion openly. In the end a lot of blue staters would eventually look at the two systems and decide to give up their dysfunctional Marxist Utopia and try their luck in the free state. The key is to have a strict and clear constitution so that we cannot waiver from liberty again.