how do i know i need to replace both catalytic converters?

This can be very straight forward or it could be BS. You will not notice any performance issues nor noise issues w/ failed o2's nor converters. For your info/knowledge ( so if they try to BS you w/ technical mumbo jumbo ) is A. Did the ecm store DTC p0420 or p430? B. Does the rear (after converter) signal stay high ? low ? or is it going high and low ? The only correct answer ( if you have a failed converter ) is the signal is going high and low. If they do not know what the rear o2 is doing then they cannot possibly know the converter is in fact bad. Having said all of this , unfortunately , Nissan converters are know to fail regularly. P.S. no mileage is stated ( and likely you are over ) but ALL converters on any make are covered by the manufacturer 8 years / 80,000 miles. P.P.S. Only the top 5% of technicians out there can determine if a converter is "weak" and 0% can tell you how long it will last ( 1 week or 5 years ). Hope this takes some of the mystery out of this process