AN ACR S80 IS SLOW TO MAKE DECISION FOR TKOF AND THEN BEGINS TKOF ROLL ONLY TO BE STOPPED BY TWR AS ANOTHER ACR B747 LANDS OVER THE S80 ON ANOTHER RWY AT ORD.

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XX25 LOCAL; ATC FREQ 120.75. ATC FACILITY: ORD. LOCATION: APCH END RWY 22L. NUMEROUS DISTRS CREATED A SOMEWHAT UNCOMMON TAXI OUT. WE HAD ACFT WITH ONLY APCH IDLE SO WE TAXIED OUT ON 1 ENG. TPS WAS INCORRECT DUE TO MEL ITEM. THE CHKLIST WAS NOT STARTED BECAUSE WE WERE WAITING FOR NEW TPS. APCHING RWY 22L; TWR ASKED IF WE WERE READY AND I SAID 'WE NEEDED A MIN.' HE ASKED IF WE COULD TAKE IT OUT IN FRONT OF COMPANY ON FINAL FOR RWY 27L. WE SAW A B747 ON ABOUT 3 MI FINAL AND CAPT SAID 'NO.' BEFORE I COULD RELAY THIS TO TWR; HE CAME BACK AND SAID 'COMPANY IS ABOUT 3 MINS OUT.' CAPT SAID 'YES.' CAPT ASKED ME TO START #2 ENG AND ABOUT THE SAME TIME TWR SAID 'TAXI POS AND HOLD RWY 22L.' THEN SHORTLY AFTER THAT HE SAID 'CLRED FOR TKOF WITH L TURN HDG 180 DEGS.' AS WE TAXIED ONTO RWY 22L I THOUGHT I HEARD TWR SAY 'ACR A; STOP.' I SAID; 'SAY AGAIN FOR ACR A.' THE ONLY RESPONSE WAS; 'STOP.' SHORTLY AFTER; AN ACR B747 FLEW OVER US AND LANDED ON RWY 27L. I CALLED TWR AND HE SAID 'YOU'RE STILL CLRED FOR TKOF; TURN L 180 DEGS.' I FEEL THE MEL ITEM COMPOUNDED BY THE LATE CHKLIST ALL DIVERTED OUR ATTN AWAY FROM WHAT WAS REALLY HAPPENING AT THE APCH END OF THE RWY. WE BOTH SAW THE B747; BUT COCKPIT OVERLOAD PREVENTED IT FROM REGISTERING IN OUR MINDS. FUTURE FLTS CAN AVOID THIS BY NOT ALLOWING DISTRS INSIDE THE COCKPIT DICTATE WHAT IS HAPPENING OUTSIDE.