LOSS OF SEPARATION OCCURRED WHEN A ZDV ARTCC RADAR CTLR DID NOT ENSURE TRAILING B190 ACR DID NOT OVERTAKE DESCENDING SAME TYPE ACFT AHEAD.

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I WAS WORKING TWO SECTORS (SECTOR 6 AND SECTOR 13); RADAR AND RADAR ASSOCIATED COMBINED. ACFT X CHKED ON SECTOR 13 AT FL240. THIS ALT WAS A WRONG ALT FOR DIRECTION OF FLT AND AN APPROVAL REQUEST WAS NOT MADE. ACFT Y CHKED ON SECTOR 13 AT FL230. ACFT Y WAS ABOUT 4 MILES IN TRAIL. I THOUGHT I SAW ACFT X AT FL230 AND ACFT Y AT FL240. I ISSUED A CLRNC TO ACFT X TO CROSS RAMMS AT 15000 AND ISSUED THE DENVER ALTIMETER. ACFT X ACKNOWLEDGED THE CLRNC. CONFLICT ALERT ACTIVATED SHORTLY AFTER ACFT X LEFT FL240. AT THE TIME OF THE CONFLICT ALERT; I WAS VECTORING TWO DENVER DEPS AND COORD WITH THE HIGH ALT SECTOR FOR HIGHER ALTS FOR BOTH DEPS TRYING TO MISS AN OVERFLT AT FL250. THE DEPS WERE IN SECTOR 6. WHEN I NOTICED ACFT Y'S ALT IT WAS FL232 AND DESCENDING BELOW ACFT Y. THE ACFT WERE ABOUT 3.3 MILES APART AND NOT IN ANY CONFLICT. TWENTY DEGS TO THE R AND I VECTORED ACFT X 20 DEGS TO THE L. THIS WAS TO TRY AND GET MY SEQUENCE TO DENVER APCH BY MOVING ACFT X TO THE TOMSN STAR. I THEN CLRED ACFT X TO INTERCEPT THE FALCON 306 RADIAL TOMSN THREE ARR. I CLRED ACFT X TO CROSS TOMSN AT 17000 FT AND ISSUED THE DENVER ALTIMETER. I THEN CLRED ACFT Y TO A LOWER ALT STEPPING HIM DOWN REF ACFT X. MY MISREADING OF THE ALTS WAS THE PROB. IF THE APPROVAL REQUEST ON ACFT X AT WRONG ALT FOR DIRECTION OF FLT WAS MADE THIS MAY HAVE HELPED TO PREVENT THE PROB. ALSO; NOT WORKING TWO SECTORS AT ONCE AND/OR ANOTHER CTLR ON THE SECTOR MAY HAVE HELPED.