ZBW CTLR TRANSPOSES DC9 AND BE30 ALT ASSIGNMENTS; DSNDS THE WRONG ACFT; CAUSING A CONFLICT.

Narrative

ACFT Y WAS AT FL210. WHEN I TOOK THE HDOF; ACFT Y WAS RIGHT ON TOP OF ACR X AT FL200; WHO I KNEW WOULD OUTRUN ACFT Y; SO I KNEW I WOULD BE ABLE TO GET ACFT Y DOWN TO 16000 FT FOR THE NEXT SECTOR. WHEN ACFT Y ENTERED MY AIRSPACE; I ISSUED CLRNC TO CROSS ALB VOR AT 16000 FT. AT THIS TIME; I BELIEVED ACFT Y WAS BELOW ACR X WHO WAS BEHIND HIM AND OVERTAKING HIM AT FL200 THIS WAS MY ERROR. I DO NOT RECALL IF I THOUGHT ACFT Y AND ACR X ALTS WERE REVERSED OR IF I LOOKED QUICKLY AT ACR X'S FULL DATA BLOCK AND INTERPED THE ALT AS FL220 INSTEAD OF FL200 OR IF THE FOR WHATEVER REASON; I DIDN'T REALIZE MY ERROR UNTIL ACFT Y'S MODE C INDICATED FL207 AND ACR X'S MODE C INDICATED FL195. AT THIS TIME; I TRIED TO SEPARATE THE ACFT AS MUCH AS POSSIBLE; EVEN THOUGH I KNEW I HAD ALREADY LOST SEPARATION. I TRIED TO STOP ACFT Y AT FL210; BUT IT WAS TOO LATE. I TURNED ACR X TO A 270 DEG HDG AND ISSUED FL180. I TRIED TO STOP ACFT Y AT FL200 AND FL190 BUT HE JUST KEPT DSNDING. ACR X LATER SAID HE RECEIVED AN RA AND THAT'S WHY HE DSNDED ORIGINALLY WHEN ACFT Y LEFT FL210. ACFT Y WAS ALSO TCASII EQUIPPED; BUT THERE IS NO EVIDENCE THAT IT WAS EVER GIVEN AN ADVISORY. SUPPLEMENTAL INFO FROM ACN 488834: I WAS UNAWARE OF AN OPERROR UNTIL THE RADAR CTLR HAD TAKEN ACTION; AND THE OPERROR HAD ALREADY OCCURRED.