Code P0420?

Hello, I have the P0420 code on my car. I have replaced the o2 sensor and have run Cataclean through it and the code has not cleared. When I called my shop, they said I have two catalytic converters, but I thought I only had one exhaust pipe. They asked me if I knew which one was the problem and are trying to get me to replace both. Do I have two? Is there a way I can figure out which of the two is bad?

I don't have symptoms like black smoke, and I don't feel like I have a hard time getting up to speed.

I'm lost and confused, please help.