For the 2013 model, you can make 2 pigtails one for ground and one for power and connect them to the leads that go to the old horn and to each of the new horns. Just make sure that if your new horns don't have a floating ground that you hook to power lead from the car to the power leads for each horn, and the ground to ground.
2014 LEAF SV
Delivery May 23 2014
50,000+ miles - all 12 bars - Same range as new - No warranty issues ever!