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.
On my prior 1997 Thunderbird, the local muffler shop refused to install my cat back 2.5" exhaust and high flow cats because it didn't have a Y-pipe cat. I explained that I had four high flow cats, he said it's not stock configuration, he can't do it. Even though it'll filter the exhaust better, he wouldn't. The car got rear ended shortly afterward, so I sold the exhaust. Now that I know exhaust isn't so bad, I'll just do it myself from here on out.
97 Thunderbird LX (The GT Bird)
PI Swapped NPI motor with some port work on the heads, PI cams, 80mm MAF, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.