The bank 1 sensor 2 is the secondary O2 sensor which is after the cat. converter in the exhaust. Usually if you replace the primary O2 sensor or Air/fuel ratio sensor in your case (bank 1 sensor 1) you'll fix the issue with the rear sensor. I wouldn't just replace both. Start with the first sensor then drive the car after clearing the code and see if you still get the bank 1 sensor 2 code.