Your front (Primary) o2 is used for the ecu to adjust the amount of fuel the car gets. Meaning, it’s the important one. If you were to run just a wideband, this would be the sensor to replace (and odds are you’d need a resistor in the original primary heater circuit to keep your car happy and free from CELs)
The rear sensor (secondary) o2 is ONLY FOR EMISSIONS purposes. It basically is looking for “clean” air. Meaning, your exhaust gases should have passed through a catalyst to clean it up before it hits the sensor. This would be the o2 sensor in which you may choose to run an o2 sim in the absence of a catalyst. (once again, to keep your car CEL free).
If you can’t clearly understand everything I just said, quit modding your car.