Those two O2 sensors are completely independent of each other. Besides P0420 almost never has anything to do with the rear O2 sensor. If you are getting P0420, that means the rear O2 sensor IS actually working! A defective rear O2 sensor will NEVER turn on P0420.
With active P0131, your engine is not running optimally and could be spewing out more emission. Determine if the wiring or front O2 sensor is bad and correct the situation accordingly. The chances are the front O2 sensor is at its end of life, so if you want to throw a part without doing any real diagnosis, put new OEM direct fit front O2 sensor.
Since you have the OBD-II, look at the front O2 voltages/graphs. If it is not switching nicely between .1 and .8 volts, or if the switching is slow or if the range is smaller, the sensor needs to be replaced. If the voltage is stuck at 0, you may have wiring problems on the O2 sensor signal line. You have the tool; use it.