Also takes 2 seconds... You don't need 3rd party apps like everyone's saying, only if you want tiling or to copy Windows behavior.

  Press Control-Up Arrow (or swipe up with three or four fingers) to enter Mission Control, drag a window from Mission Control onto the thumbnail of the full-screen app in the Spaces bar, then click the Split View thumbnail. You can also drag an app thumbnail onto another in the Spaces bar.