Hi all,
First time here was wondering if I can go from a dual manifold cat setup on my 3.3l to a single one. Reside in NJ.

Thanks in advance,

Well, first of all, you should have four catalytic converters, not two. On each bank, there will be one that bolts directly to the exhaust manifold and then the other that is part of the front pipe. The only way I could see you eliminating or modifying the front cats would be to replace them with headers; Doug Thorley or Summit Racing may have some options.
Anything is possible with a will and the money, but the short answer is "no" if you plan to keep your car on the street and pass state inspection.
First of all, removing any catalytic converter, or "gutting" them, is considered "tampering," and is illegal by Federal law and will not pass inspection in many states, including NJ. Any type of modification, such as headers, would come with a disclaimer that it is meant for "off-road use only."
The other problem will be with the ECM triggering catalyst codes. You have four oxygen sensors. On each bank, there is an upstream sensor before the front converter and another behind the converter. The ECM compares the readings from the two sensors on each bank to make sure each respective converter is working efficiently. If you remove the converter, you will have get an O2 sensor simulator, a CEL eliminator (which doesn't always work), or find some custom installation of your O2 sensors into your fabricated one converter exhaust that you are contemplating. Eliminating the post-cats and installing a performance muffler or exhaust would be a lot easier, but you still have to worry about the state inspection in regards to eliminating the rear converters. The rear converters are behind the rear sensors, so eliminating them won't trigger catalyst codes.

Thank you for the detailed response. I was under the impression that the 4 cat setup was a "California emissions" requirement. I have seen the "additional" cats in other vehicles when they were California cars.

All the parts I see on rock auto look like they have a small header and then a cat that bolts to it; what I see on this motor is an exhaust manifold combined with the cat, one piece.

Did some checking and it does look like the four cat setup is for California and on the Federal models they don't have the post converters integrated into the front exhaust pipes.

Exhaust manifold and converter:

http://www.nissanpartsdeal.com/parts-li ... erter.html

Federal 4WD exhaust:

http://www.nissanpartsdeal.com/parts-li ... m=200_A006

California 4WD exhaust:

http://www.nissanpartsdeal.com/parts-li ... m=200_A003

The Doug Thorley headers would eliminate the exhaust manifold/catalytic converter assembly...which is why it's labeled for off-road use only.

Even without the rear converters, you'll still run into the same issues I mentioned earlier.

I apologize i looked again and I see that there are bolts connecting the cats to the manifold. I will just replace the one that's tripping p0420 code and have it inspected.