That has to do with muffler capacity and type. 2x mufflers on a true dual with no merge point will be louder and raspier because you haven't collected the exhaust in some place so that sound waves can cancel certain frequencies from both banks. Some people like that card-in-spokes raspy stuff, but I think it sounds redneck as all hell on a stock cam'd truck. Remember that you have a crankshaft with throws every 90*, the way god intended on a 90* V8! Those Ford guys need to throw that flat plain crank BS right out the window because it does NOTHING but sound terrible.
Exhaust tone is totally controlled by the style of muffler used, and flow is dependent on internal passages and the case size. Volume of sound is a product of design and case size. Bigger mufflers with more internal passages will both flow better AND muffle better. You can use a single muffler with dual in/dual out and it will merge the exhaust pulses and give you a quieter and more mellow tone. The larger the case, the quieter it will be for a given design. The larger the internal passages are in said case, the better it will flow. Two separate mufflers that don't merge will be seen by the engine as two mufflers in parallel. Two mufflers on the same pipe (one after the other) will be seen as "series" and will further quiet the exhaust. With as big as these trucks are, I'm not sure why anyone would run two single mufflers in parallel when you could run 2x dual in\dual out straight through mufflers in series and really shut it up with zero flow loss.
I run 2x center in/offset out 3" perforated core straight through mufflers and my truck sounds stock while having absolutely no restrictions to flow. And it's probably no better than stock, but MDS rattles my teeth now. I wish I had never touched the exhaust on my truck.
