Multiple world spaces?

So I'm beginning to learn about networking, and I'm having trouble coming up with a solution that I see coming from a mile away. I'd like to begin understanding the solution early on before it becomes an issue.

What would I have to do in order to allow different players to be in different scenes at the same time?

Is there some built-in way for Unity to handle multiple world spaces/instances/scenes at the same time, or will it require multiple applications running and communicating with each other? If the latter, how do I get two applications to communicate with each other?