ZDV CTLR EXPERIENCED OPERROR AT FL350 DURING DSCNT OF ACFT THROUGH OCCUPIED ALT.

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I WAS WORKING SECTOR 28 RADAR ASSOCIATE POS FOR APPROX 15 MINS. THERE WAS LIGHT TO MODERATE TFC AND LIGHT CHOP OCCASIONALLY BEING RPTED. THE RADAR CTLR ISSUED A DSCNT CLRNC TO FL270 ON AN ACR X ARR AT FL360. AS THE RADAR CTLR WAS GIVING THE CLRNC; I POINTED TO AN ACR Y OVERFLT AT FL350 ON THE SCOPE. IN MY OPINION THERE WAS INSUFFICIENT ROOM FOR ACR X TO DSND BELOW ACR Y. THE RADAR CTLR THEN INSTRUCTED ACR X FLT TO MAINTAIN AT LEAST 2000 FPM IN THE DSCNT. AFTER 3 RADAR HITS; ACR X FLT STILL SHOWED LEVEL AT FL360. THE RADAR CTLR ASKED ACR X IF HE HAD STARTED DSNDING YET AND THE PLT REPLIED AFFIRMATIVE. THE RADAR CTLR ISSUED A 30 DEG L TURN TO ACR X FLT AND A 50 DEG L TURN TO ACR Y. IN ADDITION; ACR X FLT WAS INSTRUCTED TO CLB BACK TO FL360 AND ACR Y TO DSND TO FL340. SEPARATION WAS LOST. I FEEL THAT THERE WERE NO OUT OF THE ORDINARY DISTRS IN THE AREA THAT DAY. IN MY OPINION THIS ERROR COULD HAVE BEEN AVOIDED IF THE RADAR CTLR HAD USED BETTER JUDGEMENT AND NOT ISSUED THE DSCNT CLRNC; OR HAD INSTRUCTED ACR X FLT TO CLB BACK TO FL360 SOONER. I ALSO FEEL THAT A MORE EXPEDITIOUS DSCNT FROM ACR X WOULD HAVE BEEN POSSIBLE AND WOULD HAVE AVOIDED THIS LOSS OF SEPARATION.