I could be wrong but I don't think any of those Corsa exhaust systems are a "true dual", just dual from the muffler or "cat-back", at best you get dual from a single stock catalytic converter back.. They can't change the stock two into one to a true dual and sell it unless it's for off road use only because it will not pass smog in any state or county that has checks or testing.. You can do anything past the cat, just not before..
And the muffler shops are telling you it can't be done is not that it "can't be done" because it can, they just can't do it legally because it's tampering with the pollution control devices on your AV and they can be fined or lose the license. I have 2500 AV and it comes stock with 3" duals all the way back past the muffler then into a single tail pipe, with dual cats (actually quad cats, 2 per bank) so I know duals will fit on an AV..
I know it can be done, owners on this site have done it but only if they don't have checks in their state or county.. I had friends in California that lived or had properties in counties that didn't have smog checks and registered their vehicles there and did whatever they wanted with no issues until they went to sell it, then they had to make it pass or sell it "as is" and make the buyers responsibility. But that was usually at a lose..
Good luck