There are a few ways to do this, I have seen people plumb in a line for propane in both exhausts, I have seen it done with gasoline, and one other way, I forget what the 3rd fuel source is though. with all of them you need to have spark plugs in your tail pipes. From what I have heard the propane is the safest route. Just make sure you plumb in a shut off valve. I believe there are a few websites with some info on this
http://www.eatmyflames.com/http://www.flamekitz.com/http://www.flamingkit.com/that is just from a quick google search on "flaming exhaust" I do not mind assisting you or anyone else, but try to do a little leg work yourself too man. everything on the above sites is in a kit form so it would be an easy do it yourself type of deal