Welcome to Bronco Zone!!!
You might want to check the local/state regs on exhaust systems for your Bronco where you live, but I'll give you a general idea. A "true dual" exhaust set-up is hard to do on a Bronco due to the transfer case location, so alot of folks run a stock y-pipe to the 2 cats or a single replacement cat on the passenger side. From there, you can connect to a single muffler witha 2.5" inlet, and run dual 2" exits from there. Route both pipes out behind the pass.-side tire or at a 45 eg. angle at each corner of the rear bumper. NOT straight out the back, you'll get too many fumes inside the cab. A 1.5" pipe anywhere is too small, in my opinion, and a 3" before the muffler is too big (reduces back pressure). You can run a single 3" exit from the muffler.
Try a search of the forums. There's several topics and some pics of members' exhaust set-ups. I have a single cat, with a single-in/dual out Flowmaster.