ACFT BEGINS TKOF ROLL FROM BWI; MD; WITHOUT TKOF CLRNC FROM TWR.

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AS FO ON A JETSTREAM 32; I HAVE RESPONSIBILITY FOR ALL RADIO AND PA CALLS UNTIL AIRBORNE. AS OUR TAXI SPD SLOWED; AS WE APCHED THE DEP END OF THE RWY AT BWI; TWR ANNOUNCED FOR OUR FLT NUMBER TO TAXI INTO POS AND HOLD. WE HAD JUST COMPLETED THE TAXI CHKLIST AND HAD NOT YET CONTACTED TWR. I READ BACK THE POS AND HOLD CLRNC AND TOLD THE CAPT THAT I'D 'BE IN BACK' MEANING I'D BE ON THE PA AND NOT MONITORING TWR FREQ. I GAVE THE 'SEAT BELT -- TRAY TABLE' PA AND THEN TOLD THE CAPT; 'I WAS BACK;' MEANING I WAS MONITORING TWR FREQ AGAIN. HE HAD ALREADY TAXIED INTO POS AND SAID 'RUN THE BOX; LET'S GO;' MEANING FOR ME TO COMPLETE THE PRE-TKOF CHKLIST AND THAT WE WERE TO DEPART NOW. I ASSUMED THAT HE HEARD AND RESPONDED TO A TKOF CLRNC WHILE I WAS TALKING ON THE PA. HE THEN RECAPITULATED TARGET TORQUE AND THAT HE WAS 'PF.' WE HAD ACCELERATED THROUGH 50 KIAS WHEN TWR SAID 'ACR XXXX HOLD YOUR POS; HOLD YOUR POS.' THE CAPT IMMEDIATELY ABORTED THE TKOF; I LOOKED UP AND SAW ANOTHER ACFT DEPARTING THE INTERSECTING RWY APPROX 3000 FT IN FRONT OF US. I RESPONDED TO TWR SAYING ONLY OUR FLT NUMBER. AS WE DECELERATED THE CAPT TOLD TWR THAT WE'D BEEN CLRING THE RWY AT THE NEXT INTXN. WE TAXIED BACK AND (WITH CLRNC) TOOK OFF. THE CAPT MAINTAINS THAT WE WERE CLRED FOR TKOF NOT CLRED INTO POS AND HOLD. I RESPONDED TO THE POS AND HOLD CLRNC AND NEVER HEARD CLRED FOR TKOF. IT IS NOT UNCOMMON FOR ONE PLT TO BE TALKING ON THE PA WHILE THE OTHER IS TALKING ON TWR FREQ. THIS ASSURES THAT THE FREQ IS ALWAYS MONITORED AND I CANNOT THINK OF A BETTER DIVISION OF TASKS WHICH WOULD BE SAFER THAN THIS ONE. IF I HAD VERIFIED THAT TWR CLRED US FOR TKOF; THEN THE CAPT MAY HAVE RESPONDED WITH SOME OTHER ANSWER ASIDE FROM 'YES.' THIS QUICK QUERY MAY HAVE PREVENTED OUR NEAR MISS.