Dual exhaust isn't available for the 350, reason being it costs twice as much and will have negative efffects on performance on a setup like this. Very few quads benefit from duals, and the ones that do are the big bore machines with 4 or 5 valve heads and individual exhaust ports.
There are a lot of good exhausts out there, stay away from the ones that are suspiciously cheap, you get what you pay for.
HMF is a good flowing exhaust but stay away from it if you ride anywhere that the neighbors are going to be bothered by noise. That goes for a lot of aftermarket exhausts though, they're obnoxiously loud in the name of 1 or 2 extra hp's.
If I were to build another Warrior it would get a Yoshimura system on it. Not the cheapest but they build really nice exhaust systems that sound good, and with their TEC inserts you can keep the sound levels down when you need to, then when you hit the track run it wide open.