Equestria exists on its own plain, it operates under its rulers, focusses on its problems and deals with them. But what if this Equestria was not the only one, and another existed back to back. And what if a crack appeared in-between these dimensions that bleeds them together?
One Equestria: ruled by Princesses Celestia and Luna, a pair of ancient sisters who have ruled for thousands of years upon ancient thrones crafted from resourced long since depleted, adorned with crowns that have seen more life than most empires.
Another Equestria: Preparing for the coronation of their young, unprepared Princes Solaris and Artemis, two young boys who have barely made it past their seventeenth birthday, with their minds only focussed on the prospect of adventure and often neglect their duties as budding princes.
An action from both sides of this coin causes a bridge to open up between their dimensions and the young boys find themselves dragged into the other side, into a foreign, yet familiar land.