The way it has been explained to me is this. It is all about waves of pressure (sound) passing down the pipe, through the muffler (if applicable) and out the tailpipe. Without an X or H pipe, each tailpipe has four waves of sound exiting for a given amount of time. With the X or H pipe, each tailpipe has eight waves of sound exiting for a given amount of time....divided by two. The total amount of sound may be the same because there are two pipes but with twice the sound waves traveling down each pipe there is a cancellation effect. I am not a sound engineer nor do I begin to understand all the mathematics involved in effective exhaust systems. I guess it sort of "smooths out" the total sound of the exhaust. Conversely, if you have ever heard an engine with zoomies each pipe seems to be so loud because the sound is so staccato in quality, of course they are only about 18 inches long too, I guess that helps. Does all this sound weird? Maybe I shouldn't post this. Somebody save me.......
By the way, I had 3" pipes through shorty mufflers dumping in front of the rear axle on the 455 in my 1969 Delta. They were probably too big, I know they were too big to pass over the axle and exit under the bumper. That is why the dumped in front of the axle.