I think the distinction is that "2 days" is a composite which could also be constructed as "5 days", "91 days", etc.
There's no specific single word that cannot be modified which means exactly "2 days", like "uebermorgen" which specifically means "the day after tomorrow". You cannot have "5 uebermorgen", for contrast.
There's no specific single word that cannot be modified which means exactly "2 days", like "uebermorgen" which specifically means "the day after tomorrow". You cannot have "5 uebermorgen", for contrast.