Minecraft - 2nd. Directory?

I want to create a 2nd directory in Minecraft so that there's a Minecraft with mods in Forge 1.12.2 and one with mods in 1.8.9.

How does this work?

You can make it easier

-twitch launcher: you create different instances that have different worlds / mods / version and you can download modpacks like skyfsctory

-multimc: is actually almost the same

Can I do that with my own Jar files?

If you mean mods, yes you can insert them into the folder