Double strung harps have it all. Set the two rows of strings the same, and there are fun echo techniques that can only be done on a double. Quick repeated notes? No problem. Lots of accidentals? No problem. You can set, for example, a g# on the right and g natural on the left. Then you can play a passage of g-g#-a with no lever flips or pedal shifts. Unlike a cross strung harp, you can glissando in any key. Also unlike a cross strung, you don't have to learn a bunch of hand positions. Unlike a standard lever harp, you don't necessarily have to flip a lever to play an accidental. Also, you can do more with a small harp, since you don't have to worry about the left and right hands running into each other.