Now drag a window from one monitor to the other. The choice on X is to have both monitors run with the same specs (well, not for everything but yeah) or to have

Now drag a window from one monitor to the other. The choice on X is to have both monitors run with the same specs (well, not for everything but yeah) or to have two completely separate screens at the X level which means they're isolated from each other and you can't move thing between them.