here's a link. It's not performance numbers. But the idea behind the N1 dual is this. Like Drift240sxdrag said, the dual exhaust isn't on both sides of the car like most duals. It spilts after the cat, but the piping stays together straight back. Because it's a dual piping, the airflow is as fast as the N1 single which gives you the same peak HP as the N1 single does. But, because of the dual pipes are smaller in diameter, you lose less low end torque.
http://www.apexi-usa.com/exhausts_n1dual.asp