I would seriously recommend against dual exhaust for two reasons:
1. Dual exhaust on a 4 cylinder engine is not a good thing. As far as I know, u lose horsepower.
2. Dual exhaust??? That's ricey man.
But anyway, if u REALLY wanna do it, get a y pipe like redline said, and have a shop weld it for ya. If u supply your own pipes, then expect to pay anywhere from $30-$100 to weld those suckers. It really does depend on what kind of shop u go to, and how much they try to screw u over.