Do I need to replace both cats?

my 2016 Fit has an intermittent check engine light since I got it ~20k miles ago, P0420 code. I'm fairly certain the O2 sensors aren't the issue. So I'm finally giving in and gonna replace my cat :/

I've been procrastinating bc I'd just replaced the cat on my '13 Ford Fiesta a few months before buying my Fit, was very annoying to see the same problem pop up again lmfao

I know Fits have 2 catalytic converters, so I'm wondering if anyone has any insight and can help -

a) do I need to replace both? b) is there anything I should check that might've caused the cat to go out? I know it wasn't well maintained by the last owner, I suspect they racked up the 117k miles using it for uber c) how difficult it would be to do myself? I'm willing to get my hands dirty, I had to partially drop the engine to replace my Fiesta's cat lol.

Thanks !