Many years ago, there was a legend. A legend of a medallion, or should I say, two medallions. The Medallion of Light and The Medallion of Shadow. Both were separated onto two different planets called Earth. This was chosen by the gods, because they thought that it would be best. They didn't realize the special properties of the medallions themselves. They also didn't know of the special connection between the two Earths, but that's for later.
Legend also tells that the medallions themselves could be used to jump or leap to other worlds. Two heroes, from those two separate worlds went and defeated the evils that wanted to destroy and corrupt the entire multiverse themselves.
They soon returned, having both worlds balanced and both villains were defeated. Both heroes though, didn't do it just for the adventure, but also for the most powerful tools in the multiverse... Knowledge, Justice, and Friendship.
Indiana Jones of Earth &Daring Do of Equestria
Both heroes and explorers of each other's planets. Both who saved their worlds, now need to save something bigger, the multiverse, both now in trouble.
As both moved on into their lives, they didn't know what possible thing could happen next.
Then suddenly, when you least expect it... it happens.