I had this similar problem with my lexus. After 120K the code appears for both cats, I replaced 1 cat but still have one code left, then I replaced the other cat and the code still there, what I did this time was, I remove both side Osensor after the code on both sides clean them and swapped (just prevent from purchased or replaced parts that I don't need). Since I have not seen that code appear again.
I would suggest that clean them before swap them, do this for before the cats and after cats. Do this before replaced them, this way you can save costs.