Loading additional scenes? (SceneManager)

Scene & World Building Scene Management
1

I would like to have more than one scene active at runtime.
My game is a metroidvania style so how can I load in more than one scene at a time?
Since Unity 5.3 we should use the SceneManager.LoadScene / LoadSceneAsync but both of those delete the previous level I was in, how to not delete the previous scene?

Thanks for any help.

2

There is a second argument for those methods that allows you to load scenes additively.

3

Thanks yes, silly me.
LoadSceneMode.Additive