Just for kicks, is it possible to install both Unity 2D and Unity 3D at the same time without breaking either one of them? What's the difference, anyway? 2D (QT) vs 3D (GTK2)?
Yes it should be easy to install both without breakage (unlike GNOME Shell).
Whilst the 2D version is itself programmed in QT, then the underlying applications for both Unity 2D and 3D, can be QT or GTK. It makes no difference.
Yes, it's possible.
Unity 2D as far as I know is written using Qt,
the 3D version is a compiz plugin.