THE GRUMMON AMERICAN AA-5A, N9844U, HAD JUST LANDED. WHILE TAXIING, THE AIRCREW STOPPED SHORT OF RWY 13R AS BEECHCRAFT- HAWKER BH-125, N125E, WAS TAKING OFF. THE RATED STUDENT IN N125E, WHO WAS ON A TRAINING FLT, WAS AT THE CONTROLS & BEGANTHE TAKEOFF. THE POWER-UP & TAKEOFF ROLL WERE NORMAL. ROTATION WAS DESCRIBED AS NORMAL & THE L THROTTLE WAS RETARDED TO IDLE AT 110 KTS, AS PREBRIEFED, TO SIMULATE AN ENG FAILURE. THE INSTRUCTOR PLT (IP) STATED THAT THE LIFT-OFF APPEARED NORMAL & DIRECTIONAL CONTROL WAS GOOD UP TO AN ALT OF 10 TO 20 FT. THE L WING THEN STARTED TO DROP & THE STUDENT APPLIEDR AILERON, BUT DID NOT STOP THE ROLL. THE IP BEGAN ADVANCING THE L THROTTLE, BUT DID NOT GET ON THE FLT CONTROLS. THE L WING HIT THE RWY & THE ACFT VEERED LEFT & SETTLED TO THE GROUND. AT IMPACT, BOTH MAIN GEAR MOUNTS FAILED, A FUEL TANK RUPTURED & A FIRE STARTED. N125E THEN SLID INTO N9844U & BOTH ACFT BURNED. BH-125 FLT MAN RCMNDS IP FOLLOW THRU, MAX BANK 5 DEG & CAUTIONS NEGATIVE WXVANING IN X-WND. BH-125 RUDDER BIAS ENGAGED. AA-5A CREW THROWN OUT, SEATBELTS UNLATCHED