LGT GIVEN AN IMMEDIATE DESCENT FOR MLG TRAFFIC. ACFT UNDER SEPARATE ATC JURISDICTION.

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AT FL350; ENRTE DEN TO TUS; APPROX 115 NM FROM SAN SIMON VOR ON THE 348 DEG R; 'CLRED DIRECT;' ABQ ATC (FREQ 135.8 MHZ) CALLED 'LGT X; DSND IMMEDIATELY TO FL330!' I BEGAN DSCNT; RETARDING THRUST LEVERS TO IDLE AND USING SPD BRAKES AS EXPEDITIOUSLY AS POSSIBLE WITH THE SEAT BELT SIGN OFF AND PAX (3 FIRST CLASS; 72 COACH AND 3 CHILDREN) AND F/A'S ABOUT; DSNDED TO FL330 IN ABOUT 40-45 SECS. WBOUND ACFT MLG Y VISUALLY ACQUIRED AT ABOUT 10 MI; SEEN TO PASS ABOVE US SLIGHTLY BEHIND (ESTIMATED 2000') OUR VERT VIEW. CTLR DECLINED FILING RPT; STATING HAD MORE THAN 5 MI RADAR SEP AT TIME STANDARD VERT 2000' SEP ATTAINED. COLLISION COURSE NOTED BY CTLR (WHETHER VISUAL RADAR OR COMPUTER ALERTED; UNKNOWN). CREW RESPONSE TRUSTING AND IMMEDIATE; WITH POS OF TFC CONFLICT REQUESTED. SUFFICIENTLY SMOOTH THAT PAX SEEMED UNAWARE OF DANGEROUS POTENTIAL. NEED COLLISION AVOIDANCE WARNING SYS ASAP! SUPPLEMENTAL INFO FROM ACN 90420: WE COMPLIED INSTANTLY BY RETARDING THROTTLE AND DEPLOYING SPD BRAKES. I RPTED OUT OF FL350; 345; 339 AND 335; AS WELL AS FL220. MLG Y CROSSED BEHIND US LEFT TO RIGHT AT 35000'. CTLR ADVISED US ON 135.8 THAT HE WAS NOT WORKING THE SECTOR FOR THE MLG Y.