Having two cats fail at the same time on a newer vehicle is very rare - kind of like having two blowouts on the highway at the same time. Could happen if your engine burns lots of oil, but not likely at all.
Pretty sure warrantys on cats and engine computers is now 8 yrs / 80,000 miles if that helps.
Look here:
http://engine-codes.com/p2a03_nissan.html You will note that there are several causes for the code, related to the engine running lean. It COULD be a simple vacuum leak. Also note under tech notes that there is a relearning proccedure you should do.
Also note that vacuum leaks could affect BOTH cylinder banks and therefore both cats. Low fuel pressure would be the same. But a bad fuel injector will only affect the cat on ONE side. So you are looking for a problem that affects both cylinder banks.
I would buy a subscription at ALLdata which gives you access to your service manual and all factory service bulletins. See if there isn't a service bulletin on p2a03 - I seem to remember reading about a service bulletin for this code, possibly involving updates to your ECM. I'm not sure, but its worth a look. I would also call the nissan dealer and see what they may know. Tell them you did the rear O2 sensors already.
I would check your air intake for leaks carefully. If that seems ok, I would let Nissan or a good tech have a shot at it before I swapped out cats. If you just swap cats before doing the diagnostics, you may get the same code back when you are done.
. So I replaced the rear o2 sensors and left the non-foulers in... still getting the p2a03 code.
. If I could get to them I'd replace them and I think that'd fix it, but getting to the studs seems impossible... I can't even get any type of cutoff tool in there to cut them of!!!