Q&A

The exhaust system/catalytic converters are different for California and federal cars. I assume yours is a Federal car. The front what many people call an Oxygen sensor is actually an air fuel ratio sensor and all though looks similar in construction is different in operation.
The Air Fuel ratio sensor is difficult to test and people frequently do not test it correctly. It can be responsible for PO420 codes.
The rear Oxygen sensor monitors the efficiency of the catalytic convertors and is located behind the rear Cat. Convt. It should have a relatively flat inactive voltage pattern when viewed on an oscilloscope, people need to look at that pattern before condemning a cat as being bad. If they pattern is very active the cat is bad.
The inlet and out let temperature of the cats need to be looked at as another test.
The rear O2 sensor measures both cat convectors, I have seen people replace one cat and hope it fixes the P0420 code.
Your car's computer stores many problem parameters it saw when it illuminated the Check Engine Light. These codes stay in the memory of the computer until erased. Prior to just replacing parts ,you need to ensure what caused the failure is fixed. In the memory is stored Long and Short term fuel trim figures. Definitely see that these figures are +/- 3% to 5%. It is an indicator if the system is running too rich which can cause damage to the cat conv.
In California we have to fit Original Equipment cat convt. which are expensive but more efficient that many after market catalytic converters.