Pokemon South & Pokemon North
Both take place in the same region but in different areas.
8 exclusive gyms for each version.
Once you best the elite 4 you receive one of the Johto starters (now with a Mega evolution) at level 5 and can travel The other region and battle the other 8 gyms. Each of those gyms have a level restriction so you don't reuse your party (Lvl. 15 for The first one, 21 for the second one and so).
Pokemon South can be more "underdeveloped" like Hoenn and so, with a higher concentration of Grass, Bug, Fairy and Ghost types.
The North can have bigger cities like Kalos, and have more Steel, Electric, Poison and Dark types.