Is There a Mod for multiplayer in singleplayer?

Let me explain, Is there a mod where you could create a second player in singleplayer (Kinda like running 2 accounts in one PC), I know it's already possible with Alt accounts or weird Internet/file manipulation and stuff, but I feel like there would be another mod that should already exist where you could make a second player without making another MC instance (Something like pressing a button or typing a command and a new player instance is created in the world that you can switch back and forward with), but I couldn't find something like that... so I was wondering if this exists or not.