Cat back system for a 4.2 V8

As an upgrade cat back system, what is best or does it not matter?

A) From the two cats two pipes into a double centre box and then two pipes into the double back box?

B) two pipes into two single center boxes and then two pipes onto a double back box?

I'm confused

my car is at http://www.powellsperformance.co.uk/for an exhaust at the moment. I was quoted £350 all in but got a call saying they didn't reliese when quoting the job the A8 was a twin pipe system so have now upped it to £500 all in but don't have any twin back boxes in stock.

The boxes Powells use come from http://www.eurosportexhausts.co.uk/Our%20products.html