Both "meridiem" and "mediam diem" seem to have carried both the meaning "midday" and "(the) south" in Latin, if their Romance descendants are any indication.
From "meridiem", we get apparently Italian "meriggio" and Portuguese "meridio" (and English "meridian").
From "mediam diem", we get apparently Italian "mezzodi" and Portuguese "meio-dia".
Were there any differences in usage between "meridiem" and "mediam diem"? Was one more formal and one less? How did they come to be in competition?