Code p0136 fixed, now get p0156

[Traceman]

I have a 1996 Pathfinder. I had a obd II code p0136, O2 bank 1 sensor 2 (downstream). replaced the sensor, codes cleared and now I day later I'm getting code p0156 O2 bank 2, sensor 2 (downstream). Is this just bad luck and I should replace the other sensor or does this sound like something else may be going on? I have heard the rear O2 sensors are a common problem. Thanks for any help!