Lets examine the slant six (or any American six's firing order) for that matter related to exhaust pulses and scavenging.
1-5-3-6-2-4 is the firing order for every American made inline six........unless some of the new ones are different.
This means that a slant with a 6 into 2 header is perfect for scavenging.
Why? Lets split the front three cylinders and the rear three.
Keep in mind, the firing order, 1-5-3-6-2-4.
See what's going on here? The firing order alternates between the front three and rear three every time a cylinder fires. This means with every firing of a cylinder, exhaust will be scavenged from the opposite cylinder on "the other three".
So, your "theory" is not optimal. This is why a good 6 into two header and into one large exhaust pipe works best here and duals do not.
