Do you dream of an alternate Universe?

Mine is parallel and it exists simultaneously with the one that's going on. Mine is also historic rather than current.
I think of my tribe being hidden in plain sight from the evil Axis power.
I'm not quite sure how science fiction would write such a thing. Imagine two distinct worlds inhabiting the same plot of dirt at the same time. Neither of them is ethereal. They would have some of the same buildings in both worlds and the old trees and plants would also co-exist in both. How is it then, that they would/could be separated from one another?
There would need to be some measurable anomaly that causes the distortion. Maybe it's the amount of light that is allowed into one of the planes. Maybe the tribal DNA resonates with the Source and creates a shield in which they can see, interact, and recognize one another, but do not have the same with the others.
It sounds like a wonderful way to preserve a culture during hardship, famine, war.
So, what's to f**k it up? There will always be one member of the tribe who craves the others' world. He or she wants to invite the others in. My beloved other will stand with us and by us. Wither he goest, I shall go. Wither he lodgest, I shall lodge. My people shall be his people, and his people shall be my people.
And then... POOF! The universe is massacred.
That's what I sometimes think when I see old pictures of family that never made it to America.

Or... maybe I go into my time travel mode where I come to Eastern Europe with a b*tt-load of cash and tickets to get out of Dodge...