Any pipe with a Y will have more exhaust going to one side or the other. Hell... my dual exhaust car does the same thing at low rpm. Scavenging and dP dictate where the exhaust will go. Now the 90 degree bend isn't the slickest design, but it will do the job. At higher rpms, the pressure through one muffler will get high enough to make the path through the other muffler a lower dP option. That's when both will be used at roughly equal flows. Fun with fluids...
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-Keith 
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