Would venture to say the problem you're experiencing is indicative of something further up in the system that provides information to both downstream sensors. Have never seen both catalytic converters nor downstream oxygen sensors go out on a vehicle although stranger things do happen. Moreover, since they are designed to worked independently of one another, the likelihood is even less probable. Likely culprits are a vacuum leak, defective map (manifold absolute pressure) or maf (mass airflow) sensor. Have you tried cleaning the mass airflow sensor? As far as the map sensor, couldn't locate it last night, but will give it another gander in the morning.
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