>>28270300Region.
The shtick for this is that Touzai consists of two Leagues, Azuma and Nishi, sharing a single Elite Four, in the same manner as Kanto and Johto, and like in G/S/C, there is a post-Elite Four section to the game. In East Version you start in Azuma, in West, you start in Nishi.
Each game has a complete story of its own but it intersects with the story of the other at various points (like there's a shared enemy Team, for example); you'd need to play both to get the full picture.
