aturbostang said - "If you mean will you pass the visual check then yes. "
Wow - that conclusion is quite a leap by my thinking. The only one who can make a certain judgement ('yes') about whether a car is gonna pass a visual or not is the tech doing the inspection. The rest of us are just helping you guess about it. Different cats are designed to do different things. Different catalysts on the substrates in the converter help promote certain chemical reactions in the exhaust stream which help reduce controlled emissions - unburned HC, CO and NOx. If the single cat you put on is designed to help reduce the same emittants that the 2 cats you're replacing did, then the single cat should be doing what you need it to do. You're gonna need to do some research and find out more than just "single cat vs. dual cats" -- you need to know what emissions your dual cats were controlling, and what emissions your single cat is designed to control.
As for the visual, it depends on the tech. I know of situations like turbostang faced in OH where they don't even care about a visual - they just care if it passes the sniffer. I've seen other situations where the tech knew exactly what the converter configuration was from the factory, and wouldn't pass it unless it was configured that way; I've seen 'em not pass visual if the GAS CAP wasn't the proper factory piece; I've seen 'em not pass because hose running from the exhaust manifold shroud to the air filter box had a small hole in it. You just never know what you're gonna run into with your tech on that day.
i live in the same area and i put my friends MAC h pipe with 2 cats on for inspection and i passed with no problems.. although it was a friend that inspected the car so im not possative as to how strict he was with it.. i say buy an off-road h and just use the stock pipes for the inpection
Im in NY and Im pretty sure that they are tougher here then by Philly I passed fine from a person I didnt know with a high flow pipe w/ 2 cats.
Anyway if you have any problems im sure you can find a way around it. Allso you could go to the more rural areas of PA, I have friends their with vehicles so illegal its not even funny, and they just hand em a sticker
FYI aftermarket pipes with 2 hi-flo cats are designed to adequately replace the factory 4cat system. in the stock setup the first two cats are to reduce NOx (oxides of nitrogen) and the last two are called oxidizing cats which reduce HC and CO by converting to H20 and CO2. most new convertors combine these two cats in one which is why these 2cat pipes are still good. you shouldn't have a problem passing. if the tech is a dikk and fails you for 2 cats just go to a different shop.