Rainbow Dash and Twilight are sent into the multiverse by touching a shard from the Rainbow comet and they are separated as the result of it but they're not alone. At the same time, another multiverse versions of themselves also touched the shard of their own world comet. Both groups are on quest to not only get back home but also have to go through the multiverse to get to find each other but there's a problem. They don't remember each other names or why they need to find each other...Meanwhile(and the main story), other versions of themselves are showing up at the crater of comet shard site and are trying to understand what's going on.
Dash side of her adventure can be found here
Twilight Side of her adventure can be found here