I think what he's referring to is a more common setup:
Connect your positive "main" to bank A, connect your negative "main" to bank B. Couple each bank only at the end points--no blue lines that you have drawn.
The purpose of this is so that the "main" wire length is the same to each bank, and the cross-over wire is the same to each bank, once they average at.
If you connected the mains both to "bank a" and then used shorter cables to parallel in "bank b", you would effectively have a longer wire run to bank b than you did to bank a, resulting in unequal draw on the two banks and unequal service.
You could do the same thing (balanced wire lengths) by making a "Y" or using a junction block so that the main wires fed to each bank through equal lengths, but that would actually just add more components and expense.
I'll come back with ASCII art if that doesn't make sense after you've reread it a few times.