ARTCC CTLR INCORRECTLY ALLOWED AN MD80 TO LEVEL OFF AT FL310 AT THE SAME TIME A CARJ WAS SLOWLY DSNDING OUT OF FL298 FOR FL290.

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ACR Z OFF VPZ WAS APPREQUED CLBING TO FL350. ACFT WAS OBSERVED ON A SOUTHERLY HDG. ACR Z WAS THEN OBSERVED ON A SE HDG TOWARD ACR X AT FL350. I CALLED ZAU TO GET ACR Z TURNED; AND THEY SAID THEY KNEW NOTHING ABOUT HIM. I THEN CALLED ACR Z AND HE RESPONDED. I TURNED HIM TO A NE HDG AND TURNED ACR X L 20 DEGS. I THEN HEARD IND SECTOR CTLR SAY TO DSND THE ACR X. I THOUGHT I HEARD HIM SAY FL310; BUT HE SAID FL330. ACR X WAS DSNDING FAST AND ACFT AT FL310 (ACR Y) IN IND SECTOR AIRSPACE WAS BEING DSNDED TO FL290. ACR Y WAS OUT OF FL310 BEFORE ACR X LEFT FL330. DUE TO DSCNT RATES OF BOTH ACFT SEPARATION WAS LOST. SUPPLEMENTAL INFO FROM ACN 460019: MY D-SIDE TOOK A POINT OUT ON ACR Z CLBING SBOUND TO FL410. A COUPLE MINS LATER; I OBSERVED ACR Z TURNING EBOUND IN CONFLICTION WITH ACR X WBOUND LNDG STL AT FL350. I MANUALLY COORDINATED WITH SECTOR 99 (WABASH) TO DSND ACR X TO FL330 TO HELP THEM OUT. THEN I DSNDED ACR Y FROM FL310 TO FL290 LNDG STL WBOUND PLANNING ON DSNDING ACR X TO FL310 EVENTUALLY. A FEW MINS LATER; SEPARATION WAS LOST BTWN ACR Y AND ACR X. I DIDN'T OBSERVE ACR X LEAVE FL330 UNTIL HE CALLED ON AT FL310 AND ACR Y WAS OUT OF FL298. I THOUGHT THAT WABASH APPLIED A RULE WHEN I OBSERVED THE 1200 FT.