Someone must explain the technicalities involved between the two types.
I was told the duel pipes have less back pressure but the Barrett single pipe on my bike in Aus has a bigger outlet than the twin Scorpions on my bike in SA.
Both make enough noise with baffles in. The twin pipes look more balanced and pretty from the rear end. The single leaves space on the other side for tool tube. Performance wise I would venture to say there is not much if any difference. It depends more on air filters and F/A ratio settings, etc, IMHO

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2014 XT660Z - Barrett pipe, DNA stage1.5, Barkbuster hand guards and handlebars, Pivot pegs, Teknik suspension, Guard-it engine protector combo, B&B rear rack, Trax topbox.