TWO CFI'S IN A J3 CUB EACH ASSUME THE OTHER IS FLYING AS THE ACFT DESCENDS TO 200 FEET AGL BEFORE RECOVERY.

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I TOOK OFF WITH ANOTHER CFI IN THE AFTERNOON AFTER RECEIVING ROTORCRAFT TRAINING FROM THAT CFI. THE INTENT OF THE FLT WAS TO LET HIM GET SOME CUB TIME IN. I PERFORMED THE TKOF AND AT ABOUT 300 FT AGL I XFERRED CTL OF THE ACFT TO HIM. HE WAS SITTING IN THE FRONT SEAT; AND HE ACKNOWLEDGED RECEIPT OF THE CTLS. WE PROCEEDED TO TURN XWIND AND THEN DOWNWIND. WE DISCUSSED WHERE TO FLY TO AND DECIDED TO FLY OVER THE LAKE. WE PROCEEDED SOUTHWARD OVER THE CITY PARK AND OUT OVER THE LAKE. WE THEN WENT WESTWARD TOWARDS THE SHORELINE AT WHICH POINT THE PLANE STARTED A DSNDING TURN TO THE L. I ADJUSTED THE THROTTLE A LITTLE TO KEEP THE PLANE ENG FROM OVERSPEED AND MENTIONED THAT WE WERE GETTING LOW. I ASSUMED THAT HE WAS LOOKING AT SOMETHING IN PARTICULAR. I ALSO ASSUMED THAT HE WAS CTLING THE PLANE SINCE THERE WAS NO XFER OF CTLS BACK TO ME. EVENTUALLY HE MENTIONED THAT WE SEEMED PRETTY LOW. I RESPONDED WITH A QUESTION AS TO WHY HE HAD PUT US THIS LOW. APPARENTLY NEITHER OF US WAS ACTUALLY ON THE CTLS DURING THIS DSCNT. I TOOK CTL OF THE PLANE AND PUT US BACK INTO A SAFER ALT. WE THEN DISCUSSED POSITIVE XFER OF CTLS. HE HAD THOUGHT THAT I HAD THE CTLS AND THAT I WAS LOOKING AT SOMETHING OR SOMEONE ON THE GND. THERE WAS NO TWRS OR OBSTRUCTIONS IN THE AREA; BUT WE WERE LOWER THAN WE SHOULD HAVE BEEN; AND I COULD HAVE TAKEN THE CTLS IF THERE HAD BEEN A SERIOUS SAFETY ISSUE. THE NATURE OF A PIPER CUB IN BEING A TANDEM ACFT WITH THE PAX IN THE FRONT OF THE PLANE MAKES POSITIVE XFER OF CTLS CRITICAL AS THE INSTRUCTOR OR OTHER PLT CANNOT READILY SEE WHAT THE OTHER PERSON IS DOING AND WHETHER THE CTLS ARE BEING MANIPULATED. WE TRAVELED BACK TO ZZZ DISCUSSING THE SITUATION THAT HAD JUST OCCURRED. WE BOTH VOWED TO BE BETTER AT POSITIVE EXCHANGE OF CTLS. I PROCEEDED TO LAND THE PLANE AND DROPPED THE PAX OFF. BOTH PLTS ARE ACTIVE CFI'S ALTHOUGH THE PAX WAS PRIMARILY A HELI CFI AND WAS NOT VERY FIXED WING CURRENT. WE WERE BOTH WELL RESTED AND HAD RECENTLY HAD LUNCH. I DON'T BELIEVE THERE WERE ANY AERO MEDICAL FACTORS INVOLVED.