Since you ask for a book, I suppose you can read music. Why don't you search at your library or a music store for duets for flutes or recorders? Recorder music is often in F, but you could write it out in a new key.

The people you speak of, did they play two D whistles, the high-pitched ones? Because they can get shrill, and two of them at once could be very tricky to play in tune. Especially the high notes. Of course, some professionals have very expensive 'penny' whistles.

Do you have a low whistle? A duet between and high and a low would probably be quite lovely.