SW4 TRAINING FLT STOPS WHEN OBSERVING B757 BEGIN TO TAXI ONTO RWY SIMULTANEOUSLY AFTER RECEIVING A TAXI INTO POS AND HOLD CLRNC IS RECEIVED FROM MIA TWR.

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I WAS TAXIING THE ACFT AS INSTRUCTED VIA TXWY P FOR DEP TO RWY 27L AT MIA. AS WE APCHED THE HOLD SHORT; I SWITCHED TO TWR FREQ 118.3. A B757 WAS ALSO TAXIING FOR THE SAME RWY. THE TWR CALLED OUR CALL SIGN TO TAXI INTO POS. AS THE B757 WAS AHEAD AND TO OUR L; I CALLED TO CONFIRM 'CLRNC TO TAXI INTO POS.' TWR RESPONDED 'YES; TAXI INTO POS.' THINKING THAT WE WERE TO DEPART AHEAD OF THE B757; I ROLLED OVER THE HOLD SHORT LINE AND THEN NOTICED THE B757 WAS ROLLING SLOWLY FOR TKOF. THE B757 CREW THEN KEYED UP AND SAID TO US HE 'BELIEVED' CTLR WAS PUTTING US AHEAD OF THEM. MY ACFT WAS STOPPED WELL CLR OF THE DEPARTING B757 AND THEY; SEEING US; STOPPED; THEN CONTINUED FOR TKOF. THE GND CTLR NEVER INSTRUCTED US TO SWITCH TO TWR OR IT WAS BLOCKED. AS I NEVER HEARD THE B757 CLRED FOR TKOF OR INSTRUCTED TO TAKE THE RWY; I BELIEVED THAT THE HVY ACFT WAS TO HOLD AND WE DEPARTED AHEAD OF HIM. THE B757 CREW WAS ALSO CONFUSED. THIS SIT WHERE A DEPARTING ACFT IS CLRED FOR TKOF WHEN UP FIELD OF AN ACFT IS NOT UNCOMMON AND OCCURS AT THE INTXN OF RWY 9L AND TXWY L1. THE SIT WAS NEVER A DANGER TO EITHER ACFT. THIS COULD BE PREVENTED IF THE TWR WOULD INSTRUCT THE ACFT TO 'TAXI INTO POS BEHIND THE DEPARTING ACFT' OR 'AFTER THE DEPARTING ACFT CLRS; YOU ARE TO TAXI INTO POS.' IT IS MY HOPE THAT MY DESCRIPTION OF THIS HELPS OTHERS TO GAIN INSIGHT TO THESE CONTRIBUTORY FACTORS AN ENHANCES SAFETY.