INST PLT AND STUDENT PRACTICING SINGLE ENG APCHS TO STK IN A PA23 STALL; SPIN AND RECOVER AT 100 FT.

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2 QUALIFIED PLTS WERE IN THE FRONT SEATS. 1 PLT (THE OWNER) IS MULTI ENG LAND CERTIFICATED AND A SINGLE ENG LAND CFI. THE OTHER (ME) A MULTI ENG LAND INSTRUCTOR. THE FLT WAS BEING CONDUCTED TO INCREASE PROFICIENCY IN THE PIPER APACHE AND TO SATISFY THE OWNER'S INSURANCE REQUIREMENTS. THE CREW HAD FLOWN TOGETHER PREVIOUSLY; 4 PREVIOUS FLTS IN THE PA23 AND A FEW OTHER FLTS IN SINGLE ENG AIRPLANES. THE INSTRUCTOR BELIEVES THE OWNER TO BE A GOOD PLT. AFTER FLYING TO STK; AN UNEVENTFUL LNDG WAS MADE WITH A SIMULATED ENG OUT (BY USING ZERO THRUST ON THE ENG DEEMED TO BE INOP). A TKOF WITH BOTH ENGS WAS MADE AND ANOTHER LNDG WAS BEING PREPARED. AS THE OWNER SET UP THE CTLS FOR THE NEXT SIMULATED ENG OUT LNDG; THE INSTRUCTOR VERBALIZED THAT THE SPD WAS GOING BELOW BLUE LINE (VYSE) AND THE OWNER ACKNOWLEDGED. SPD WAS STILL ABOVE RED LINE (VMC). THE OWNER MADE A FEW MORE CTL INPUTS AND STARTED A L XWIND TO DOWNWIND TURN; EXACT SEQUENCE OF EVENTS IS UNKNOWN; THEN THE APACHE ROLLED TO THE L NOSE DOWN FROM PATTERN ALT. BOTH PLTS IMMEDIATELY TOOK CORRECTIVE ACTION: PWR OFF; AILERONS NEUTRAL; RUDDER OPPOSITE; ELEVATOR FORWARD THEN AFT. CTLED FLT WAS REGAINED 200 FT AGL. LESSONS LEARNED BY THE INSTRUCTOR: DO NOT BECOME COMPLACENT WITH THE CTLS WHEN ONE IS FLYING WITH ANOTHER THAT THE INSTRUCTOR BELIEVES TO BE GOOD. KEEP A HAND ON THE THROTTLE QUADRANT TO MAKE SURE THE CTLS ARE BEING POSITIONED PROPERLY. WE HAD A THOROUGH DEBRIEFING AFTER THIS INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISES THAT THE ACFT IN USE WAS THE LOWER PWRED 150 HP VERSION AND; ALTHOUGH EQUIPPED WITH TURBO CHARGERS; THEY WERE NOT IN USE AT THE TIME. HE ALSO ADVISED THAT ALTHOUGH THEY WERE FLYING A L-HAND PATTERN; HE CHOSE TO SIMULATE FAILURE OF THE L ENG WHICH NECESSITATED EITHER TURNS INTO THE DEAD ENG; THE PROX ULTIMATE CAUSE OF THE STALL/SPIN; OR A REQUEST TO THE TWR TO MANEUVER SO AS TO UTILIZE ONLY R-HAND TURNS TO ALIGN WITH THE RWY.