As the name suggests - a double flute consists of two flutes (first one playing melody, second one holding a steady note/drone which always harmonizes with the melody) joined with one mouthpiece which-when played together-bring that specific, haunting and mystical sound.
They can still be played separately as well so you can use double flute as a simple single flute.
To make two branches play in tune with one another doesn't happen easily and sometimes ends up as an expensive firewood.