A DSNDING B727 OVERSHOOTS ITS ASSIGNED ALT WHEN THE CAPT BECOMES DISTR AND MENTALLY ACCEPTS ANOTHER ACFT'S CLRNC TO A LOWER ALT W OF CVG; OH.

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ESTABLISHED ON A R DOWNWIND FOR RWY 18R AT CVG; ATC CLRED US TO DSND TO 7000 FT. DURING OUR DSCNT; ATC CLRED ANOTHER COMPANY FLT TO EXPEDITE FROM 7000 FT TO 4000 FT. WE WERE PASSING 10000 FT WHEN THE CAPT EXTENDED OUR SPD BRAKE TO INCREASE OUR DSCNT. AT 8000 FT; THE CAPT NOTICED TFC AT OUR 12 O'CLOCK POS AND LOWER. THE TFC DID NOT APPEAR ON TCASII. I THEN HEARD THE FO CALL OUT '7000 FT; 7000 FT.' I LOOKED AT THE CAPT'S ALTIMETER AND OBSERVED US DSNDING BELOW 7000 FT AS THE CAPT ARRESTED OUR DSCNT; ADDED PWR; AND RETRACTED THE SPD BRAKE. LOWEST INDICATED ALT WAS 6650 FT. NO INQUIRY WAS MADE FROM ATC AND WE QUICKLY RETURNED TO 7000 FT. I BELIEVE THAT THE TFC THAT WAS AHEAD OF OUR FLT PATH PROVIDED ENOUGH DISTR AT A CRITICAL TIME AND CAUSED A SCAN BREAKDOWN TO OCCUR. SUPPLEMENTAL INFO FROM ACN 475021: ATC WAS TALKING WITH COMPANY FLT AND ASKED HIM TO EXPEDITE HIS DSCNT THROUGH 7000 FT TO 4000 FT. WE CHKED ON FREQ; BUT GOT NO REPLY. I DON'T KNOW WHY; BUT I WAS NOW EXPEDITING MY DSCNT THROUGH 7000 FT FOR 4000 FT. I EXTENDED THE SPD BRAKES AND WAS DSNDING THROUGH 8000 FT AT 2500 FPM.