CPR Y DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACR X. SYS ERROR.

Narrative

ACR X AT FL200; WAS SWBOUND. CPR Y WAS SEBOUND AT FL210 LNDG MDW. ACFT #3 WAS NWBOUND DSNDING TO FL220. I ISSUED TFC TO ACFT X AND Y. I THEN LOOKED AT ACFT #3 WHO WAS LNDG AT DBQ AND STARTED WORKING OUT MY PLAN TO GET HIM DOWN. I DECIDED TO TAKE HIM UNDER ACR X AND OVER CPR Y. AFTER I HAD CALLED TFC FOR ACFT X AND Y; I HAD SWAPPED THEIR ALTS IN MY MIND; THINKING ACR X WAS ON TOP. I ISSUED A CLRNC FOR CPR Y TO FL170; HDG 110 DEGS. I THEN LOOKED AT #3 TO SEE HOW THE PLAN WAS LOOKING NOW. I LOOKED BACK AT X AND Y TO SEE THE CONFLICT ALERT ACTIVATE AND SEPARATION ALREADY LOST. I ISSUED CPR Y FL210 BUT AS I WAS GIVING THE CLRNC; SAW THAT HIS MODE C ALREADY SHOWED FL203. I DIDN'T WANT TO SEND HIM THROUGH ACR X ALT TWICE; SO I REISSUED A CLRNC TO FL190 EXPEDITE THROUGH FL200. THE DIRECT CAUSE OF THE ERROR WAS MY SWAPPING THE 2 ACFT'S ALTS IN MY MIND. THE ERROR COULD HAVE BEEN AVOIDED BY CHKING THE RADAR 1 MORE TIME BEFORE ISSUING THE CLRNC TO FL170. TFC WAS VERY LIGHT (4 ACFT ON FREQ); AND I WAS PROBABLY TOO COMPLACENT ABOUT THE TFC SIT.