Hanging your picture with just one hanging wire may be cost efficient but it’s not the safest method. The artwork can easily tip out of level and possibly slide off if bumped by a passer-by or blown by a breeze. With two hangers and hooks the artwork stays level and hangs flatter against the wall.
Slipping the two hooks into D-rings makes adjusting the artwork to get it level easy. With just one hanger affixed to the wire at the back of the picture, it’s hard to get the picture straight as the hook slides on the wire.