dual exhaust

This comes up periodically, especially in more active/less permanent groups so I figured I should make sure it's posted somewhere with better long term search potential. As far as I can tell from online exhaust calculators and other discussions a single 2" straight pipe is barely enough for...

So I was driving down the highway recently and my muffle fell off after I inched home I take a look under the car I notice that the flange on the mid-pipe is missing a side and the one bolt holding the muffler on fell off. That explains it coming loose but it shouldn't straight up fall like it...