I really like both Unity and Unity 2D. They currently have different issues, meaning that one does not have the issue the other has. For instance, when I play Battle for Wesnoth, then it integrates nicely with the global menu on Unity 2D, but not so well on Unity. So I tend to switch to Unity 2D while playing it, since I can then use my other monitor without switching between windowed and fullscreen mode. There are other cases as well.
However, for me, that currently means logging out and back in. I know that this isn't necessary, so I was wondering exactly what to do to switch between them in-place, without logging out.