ZID VECTORING TWO B757'S DO NOT MAINTAIN STANDARD SEPARATION BTWN ONE IN DSCNT AND THE OTHER AT LEVEL FLT.

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I WAS WORKING HUF 'R' POSITION BY MYSELF AND WAS SPACING FOR CVG. THE SECTOR ABOVE WAS EXTREMELY BUSY AND WAS HANDING ACFT OFF TO ME LATE FOR STARTING THEIR DSCNTS TO CVG. ALSO I HAD SEVERAL DEPS THAT HAD TO BE LEVELED OFF IN MY SECTOR THAT WERE SCHEDULED TO CLB TO THE SECTOR ABOVE ME. THIS REQUIRED EXTRA COORDINATION AND VECTORING. I TOOK A HANDOFF ON AN ACFT FROM ZAU AT FL230 ON THE TARNE ARR. I ALSO HAD A HANDOFF FROM THE SECTION ABOVE ON AN ACFT DSNDING TO FL240 ON THE TARNE ARRIVAL FROM THE W. I DSNDED THE FIRST ACFT TO 17000 FT. I TOLD THE SECOND ACFT TO FLY PRESENT HDG; VECTORS FOR SPACING. HE SAID HE WAS IN THE TURN AND WHAT HDG WOULD I LIKE. I SAID HDG 180. I CONTINUED WITH THE OTHER ACFT DUTIES. WHEN I LOOKED BACK; THE SECOND ACFT LOOKED LIKE HE WAS ON A 10 DEG HDG. I TOLD HIM TO FLY HDG 240 AND ASKED IF HE GOT THE 180 ORIGINAL HDG. HE SAID HE DID; BUT HE HAD ALREADY STARTED A L TURN FOR THE ARR BEFORE I ISSUED THE R TURN TO 180. I TOLD HIM TO MAKE A HARD R TURN TO THE 240 DEG HDG. SEPARATION WAS LOST BETWEEN THE 2 ACFT.