The two catalyst system is common to some cars. But this usually makes them last longer. The O2 sensors are the only data that anyone uses today unless the drag out the emissions sensor dino from the closet. Try the O2 sensors to start. Also check for bad fuel injectors. I normally replace all FI at 100k but you did not say your miles. Fuel injectors can be as much as 60$ a piece but for mileage they make your economy and save the catalyst.