How to make different leaders rule in same year in different scenarios?

[Code Engineer 19]

I want to make alternative history scenario where leaderY came up instead of leaderX. But how to make leaderY rule only in my scenario and leaderX to rule in default scenario while both scenarios use same year? I don't wanna to do it through event.

[Łukasz Jakowski 13,395]

use government type while naming the file