Anyone know if car would pass smog with a true dual exhuast system in San Diego??? would i have to have to O2 sensors one for each side, or would adding aontehr CAT be a probelms since orignially the GTA is single CAT????
Headers > CAT> Muffler

Technically no, since CA in its stupidity does not allow adding a cat even though it would make the engine cleaner.

You can go y-pipe into single cat, into dual pipes, but why? There's barely any room under there and the performance gains have not been proven. A good mandrel-bent 3" pipe or even 3.5" pipe is enough flow for most street engines.