Pertaining to CA specifically,
, when I did my smog check on my Bird earlier this year, I asked my smog guy if doing a dual exhaust system is CA legal. He informed me that as long as there were primary and secondary set of CATs (CA approved high flows of course) on each side, that the car will still pass the visual inspection and will likely pass the sniffer test as well.
That said, when it comes down to doing my exhaust, I know how I'm going to do it! LOL.