The PUT sensor is what actually tells the car how much boost to make so the piggyback uses that one to fool the ecu into making more boost the manifold map sensor post throttle is used for temperature and fueling assistance to the O2s. if you only did the manifold sensor it couldn’t get the ecu to request more boost and if you only did the PUT sensor the ecu would see 2 big uncoordinated variances in boost pressure.