On applications with two cats on each side, the front cat is a "pre-light" unit. It's designed to run at a hotter temperature to burn off excess hydro-carbons that may be in the exhaust.
The 2nd cat is the one that does the heavy lifting (SMOG wise).
Here's the problem. If the pre-light cat is NOT working, then the unburnt hydro-carbons in the exhaust will make the 2nd cat run too hot. This will ruin it.
There's a saying that goes, "cats never die, they are murdered". That means a cat should last the lifetime of the car. Unless something external causes cat failure.
Examples include, too rich, too lean, excessive oil in the exhaust, coolant in the exhaust, or anything that make the cat run too hot.
SO.... The challenge is to rule out an external cause for the cat failure. Otherwise the risk is premature failure of the replacements.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.