Comment 7 for bug 737324

The only way to "fix" the issue when installing both meta packages at the same time would be to move one of the depends to a recommends. Though that'd only work if one of the desktop can work properly with the notification daemon of the other.

So if xubuntu can work without xfce4-notifyd and use notify-osd instead, moving xfce4-notifyd to recommends instead of depends would do the trick.

Otherwise, I think the best, if possible would be to prompt the user on upgrade telling them that the upgrade can't continue because they have two different desktops installed at once and ask them to choose which to keep. I don't know how easy it's to do that in the upgrader tool, mvo would probably know.

Of course, that upgrade step should only apply when we have two -desktop packages that can't be installed at the same time. It's normal for edubuntu-desktop and ubuntu-desktop to be installed at the same time (as edubuntu-desktop depends on ubuntu-desktop).