Harpists, by nature, are solitary people who love horses. Getting harpists to play fast together and contour a melodic line is neigh-on impossible. So we have to give all the notes to one person so it doesn't sound like harp hell. But now you need to rest, because plucking like that is insane, so the other person gets the next tough part.
You also need to make sure that each part makes sense so you can practice it and not feel like you're throwing your soul down a well.
I believe this is the best arrangement of this piece for 2 harps; A thing which should never have been tried. The arrangement itself is in the Public Domain, and sheet music is available.