I am two

Double in the night
Single in the day,
Why now am I half of what I could have been,
should have been

Am I one,
or two
or half
or none

Where am I,
Where am we,
Where are I,
Where are we?

When part of me is separate,
which one am I? (or is it we?)

Where is He
Where is She
How many am I
And where could it be?