In airtrek we have only 1 just before the cat converter. We can fix another o2 sensor on the turbo elbow outlet just after the exhaust manifold but that o2 sensor is usually a wideband sensor used for tuning or an AFR gauge. The 1 just before the cat converter is a narrowband o2 sensor used by the ecu to adjust the AFR of the car automatically, of course that's if your tuner did not bypass it when tuning your car.