How can I open Minecraft several times?

I built a circus in my Minecraft world. I always have to change my skin to play a different role. Now I want to know how to open Minecraft several times and if you can play with different skins. Can one be in a world several times?

I'm looking forward to your responses.

No you can't be in a world several times

Since you prefer to get a mod, with which you can change the skin on the fly.

OK. Can I open Minecraft several times? If yes how?

Sure, you can have Minecraft open several times. Playing with different skins will be difficult, but it should also be possible.

Do you know a mod that allows me to change the skin on the fly?

No, but you'll most likely have to use Forge.