FOREIGN ACR AND ANOTHER ACR WERE AT SAME ALT WITH LESS THAN STANDARD SEPARATION. OPERATIONAL ERROR.

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ACR X DEPARTS SJU FOR JFK AND WAS ISSUED FL280. X READS BACK FL280. LATER X WAS ADVISED FL280 WOULD BE FINAL ALT. ACR X READBACK WAS FL310. I; AS THE CTLR; DID NOT HEAR THE FL310 READBACK. ACR X REPORTS LEVEL; I THOUGHT I HEARD FL280. I TERMINATED RADAR SERVICE AND CHARGED X FREQ TO ENROUTE COMMERCIAL RADIO. ACR X LEAVES RADAR COVERAGE TO NONRADAR ENROUTE AT FL310. THREE TO FOUR MINUTES IN FRONT ON SAME ROUTING IS ACR Y; ALSO AT FL310 SEPARATION NEEDED AT THIS TIME IS 15 MIN. WHEN ACR X REPORTS TO COMM RADIO THE SECTOR 4 NON RADAR CTLR IS MADE AWARE OF ACR X'S ACFT; AND HE REROUTES X TO ACHIEVE STANDARD SEPARATION. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: RPTR STATES THAT OP ERROR REVIEW DID NOT REVEAL THE REASON THAT FLT CREW OF ACR X THOUGHT THEY WERE CLRD TO FL310. HE HAD TOLD X TWICE THAT ALT ASSIGNMENT WAS FL280. WERE GETTING READY FOR DAILY SBOUND AND ALL ALT ON A300 WERE OCCUPIED SO WHEN ERROR NOTED THEY MOVED ACR X OVER TO AIRWAY A-23. BELIEVES ACR X AND ACR Y WERE AT SAME ALT FOR JUST A FEW MINUTES AS HIS D-MAN CAUGHT IT WHEN COM RADIO GAVE REPORT. RPTR SAYS THAT HE DID A POOR JOB OF LISTENING TO READBACK AND ALSO FAILED TO CHECK MODE C ALT READOUT PRIOR TO CHANGING ACFT OVER TO COM RADIO.