Baz,
I can see your point, but the two valleys have always been regarded separately. If they didn't both have "Langdale" in their titles it'd be easier. In some books the site is refered to as "Langdale", in others as "Great Langdale". In books of a more local nature "Great Langdale" is always mentioned.
It's definitely more important to define them as Little or Great within Cumbria.
Cheers,
TE.