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The N54 uses a typical inline 6 firing order. In cylinder order, they fire in this sequence, cyl 1, 5, 3, 6, 2 ,4. That means that a true dual exhaust split between cylinder groups 1-3 and 4-6 will provide the best possible scavenging (I believe that's true for turbos as well).
The 120 degree separation of firing pulses in the two banks is the reason for that effect (120 degrees of crank rotation -think of it as a unit of time). The exhaust pulse is creating a vacuum behind it which helps scavenge.
Someone with better knowledge of 'exhaust velocity in a 6 cylinder inline engine' should chime in here.
That said, I do believe that the further back you can keep this exhaust a true dual the better it will be (and Garrett you were onto this one - and see I do have some engineering knowledge ; -).
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