I was logging some PIDs today and noticed that at a stop the B2S2 sensor voltage drops to below .20 while the other hangs out around .70. While driving they basically stay in sync.
Is that indicative of a bad cat?
yes
the rear o2s should switch slow and take a while to go lean
Correct. Since you did replace the sensor, and still getting the same issue. B1S2 is a good reference as to how it should react. If rear oxygen sensor is switching from rich to lean at a somewhat fast frequency. Most likely it is because cat has stopped working properly.