I'm a bit confused about this one. Does Ubuntu use two different window managers at the same time?
They're not used at the same time but I can see the cause of the confusion.
- Metacity handles lightweight composting and also does window decorations.
- Compiz only does the window management garb and hands off window decoration to
emerald. By default
gtk-window-decoratoris used to emulate the current Metacity theme but it also has its own cairo-based themes.
So while they might look similar in some aspects, apart from the actual theme files used, they're very different underlying code.
But why have both at all?
Because Compiz needs better hardware to run. Lots of people still don't have hardware that supports DRI, so Ubuntu includes both to give everybody a desktop.
You can only use one window manager at at a time. As psusi has written Ubuntu chooses between Metacity and Compiz depending on your settings.
Metacity and Compiz are just the two window managers available in Ubuntu's standard setup but there are many more available and most of them can be used with Gnome. xwinman gives a good overview on what's available and has a short explanation on the general concepts behind window managers.