Here's a PM from Spridget about adding dual exhausts:
Hello,
My '99 SE came with single exhaust. To add duals was very simple. I got the back half of the dual exhaust from a junk yard. I chopped off the OEM mufflers and added MagnaFlows. I also needed the right side exhaust hanger which I aquired from the junkyard. I removed the original single exhaust and laid the rear split over it and cut the single where the dual would line up. I hung everything and then welded the two sections together. Clamps would have worked too.
The sound. I think initially I was not impressed with the sound of the exhaust. First of all, it was not what I was accustomed to hearing. Secondly, I had nothing to compare it with before hand to have an opinion. But after about 2 weeks the sound improved. Some mufflers need time to settle in, and that's what the MagnaFlows did. I'm actually quite happy with their tone now. I would recommend them with the warning, they may sound strange at first, but once they settle in they sound good. More tone, still relatively quiet, subdued, some cabin noise, muffled rumble at low RPM's.
Performance. The Magnaflows are a straight through design, no baffles, no chambers. They are essentially large resonators. Resonators are straight through with small louvres inside the inner bore and mesh (stainless or fibreglass) in the outer bore to cancel out the sound waves. Flowmasters are chambered, but have a popular sound. Borlas are straight through. I do not have any evidence to show a performance increase, but I believe the engine does feel easier to rev, especially in higher RPMs. Quasi dual exhaust is mostly for looks. It adds additional weight with very little performance increase, if any. True duals (which would require custom work) would be better, but hard to build for this car for several reasons. In my opinion, the best performance exhaust for a stock engine would be a custom mandrel bent 2.25" single exhaust with a straight through muffler. With engine upgrades, 2.5" and 3" for smaller turbos. Stock exhaust on 96-99's is 2". 2000+ 24valve engines use 2.25" and as far as I know, the 2000+ exhaust is a direct swap with the 96-99 models with the same engine.
trying searching in the performance forum for more info. Also check the Archives..... lots of good stuff in there
-luke
Hope that helps.