i have dual pipes (each with a cat and muffler) on my 400.
That wasn't stock -- it was somebody's attempt to pacify some local-yocal inspection. The EPA never certified a dual-converter factory setup on a gen 2 F-body.
That was my attempt at some local-yocal inspection!
i know it wasn't stock. When I got the car it had straight pipes off of the headers with only muffs, no cats. Delaware made me get cats so I opt out of a y-pipe and just put a cat on each side.