THE FLIGHT CREW FAILED TO ADHERE TO APPROPRIATE PROCEDURES AND DIRECTIVES WHEN THEY FAILED TO SELECT THE HP COCK LEVERS TO THE HSWL (LOCK OUT) POSITION WHILE PERFORMING AN APPROACH TO LANDING STALL DURING A TRAINING/TEST FLIGHT. WHEN STALL RECOVERY WAS INITIATED, BOTH PROPELLERS 'HUNG UP' WHEN THE HIGH (CRUISE PITCH) STOPS OF EACH PROPELLER FAILED TO WITHDRAW. AS THE POWER LEVERS WERE ADVANCED, TURBINE GAS TEMPERATURES (TGT'S) EXCEEDED LIMITATIONS; THE LEFT PROPELLER AUTO-FEATHERED, THE RIGHT PROPELLER WAS LATER FEATHERED BY THE CAPTAIN. RESTART PROCEDURES WERE ATTEMPTED WITHOUT SUCCESS, AND A FORCED LANDING IN A PLOWED CORNFIELD ENSUED. EXAMINATION OF THE ENGINES REVEALED THAT THE TURBINES HAD BEEN 'SUBJECTED TO SEVERE (AND DESTRUCTIVE) THERMAL DEGRADATION DURING OPERATION' AS A RESULT OF THE PROPELLERS BEING CONSTRAINED DURING LOW SPEED OPERATIONS. TESTING OF THE RELAYS REVEALED THAT THE HIGH STOP WITHDRAWAL RELAY FOR THE RIGHT PROPELLER FUNCTIONED INTERMITTENTLY.