PLTDEV WHEN ACR Y RESPONDS AND TURNS TO HDG ASSIGNED TO ACR X. ACR Y DID NOT SEE COMPANY IDENT WHEN HDG WAS REPEATED. ACR X DID NOT CHALLENGE THE CLRNC. RPTR WAS NOT AWARE THAT ACR X HAD NOT ACKNOWLEDGED THE CLRNC UNTIL ACR Y'S TARGET WAS OBSERVED TURNING. LTSS.

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I WAS SEQUENCING JET ARRS TO EWR. I BUILT A GAP TO PUT ACR X IN SEQUENCE. I ISSUED A XING RESTR TO ACR Y TO CROSS PXT AT FL290. I DSNDED ACR X TO FL280 FROM FL330. 1 MIN LATER; ACR Y RPTED LEAVING FL370 FOR FL290. BOTH ACFT WERE CONVERGING ON PXT. ACR X WAS NOT DSNDING VERY MUCH. I ISSUED A 040 DEG HDG FOR DSCNT AND ASKED ACR Y HIS HDG. HE REPLIED 030 DEGS. I THEN ISSUED ACR X A 020 DEG HDG TO ENSURE SEPARATION. I MISSED THE READBACK. (AT THIS TIME THE ACFT WERE 8 MI APART AND ON DIVERGING HDGS.) ACR Y HAD ANSWERED AND TAKEN ACR X'S HDG. AFTER 2 UPDATES ON MY DISPLAY; I NOTICED ACR Y TURNING AT ACR X - - BOTH AT FL300 AND DSNDING. I IMMEDIATELY TURNED BOTH ACFT AWAY FROM EACH OTHER. AFTER REVIEWING THE AUDIO TAPE RECORDING I WAS SHOCKED TO HEAR THAT IT WAS ACR Y WHO READ BACK THE 020 DEG HDG AND NOT ACR X. HE DID NOT USE THE 'ACR Y' IN HIS CALL SIGN. IT WAS A CLR XMISSION TO ACR X; AND THEY DID NOT EVEN QUESTION IF THE CLRNC WAS FOR THEM. I BELIEVE THE CLASSIC HEAR WHAT YOU WANT TO HEAR IS WHY I MISSED THE READBACK; BUT IF ACR Y ANSWERED THE TURN FOR ACR X BY SAYING ACR Y; I MIGHT HAVE HAD A BETTER CHANCE TO HEAR THE MISTAKE AND IMMEDIATELY CORRECT IT. I WAS ONLY WORKING 4 ACFT ON THAT FREQ.