Comment 6 for bug 1074314

Interesting. In detail:

/usr/share/applications/gnome-datetime-panel.desktop has OnlyShowIn=GNOME;
/usr/share/applications/indicator-datetime-preferences.desktop has OnlyShowIn=Unity;

So this prevents both displaying at the same time. In Gnome classic it's possible to choose between the applet (configured by the former) and the indicator (configured by the latter) or even have both at the same time. I agree that there isn't a good solution for this since if for some reason you add both you need both control panels, but this would require the control center to be aware of what you had on your panels.