LESS THAN STANDARD SEPARATION WHEN FASTER ACR-MLG OVERTOOK ACR-LTT WHILE DESCENDING TO THE SAME ALT.

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I WAS INSTRUCTING A TRNEE ON THE N ARR SECTOR. LTT X CHKED IN AT 8000' AND WAS TOLD TO DEPART ROBRT HDG 180 DEGS AND EXPECT A VISUAL APCH TO RWY 1R. LTT X WAS DSNDED TO 7000'. MLG Y CHKED IN AT ROBRT SHORTLY THEREAFTER; WAS TOLD TO FLY HDG 170 DEGS; DSND TO 7000' AND EXPECT A VISUAL APCH TO RWY 1R. I INDICATED TO THE TRNEE THAT I WAS NOT COMFORTABLE WITH HIS INSTRUCTIONS FOR MLG Y TO DSND TO 7000' UNTIL HE WAS SURE THAT LTT X HAD VACATED 7000 FOR 5000'. WE DISCUSSED RATES OF DSCNT OF THE 2 ACFT AND I WAS SURE THAT LTT X HAD ALREADY BEEN DSNDED TO 5000' VOL OF TFC WAS MODERATE AND I WAS WATCHING THE ALTS AND HDGS OF OTHER TFC WE WERE SEQUENCING FOR THE E DOWNWIND. ALSO; THERE WAS AN EBND ACFT BEING WORKED BY ANOTHER SECTOR WITH SCRATCHPAD ALT INFO INDICATING THAT IT WAS DSNDING OUT OF 6000 FOR 2000'; HDG TOWARDS OUR AIRSPACE AND WOULD HAVE CONFLICTED WITH AN LGT WHO WAS DESCENDING TO 5000'. A POINT OUT WAS GIVEN TO US ON THE EBND ACFT AND TFC WAS CALLED TO THE LGT. I TURNED MY ATTN TO LTT X AND MLG Y. THE DATA BLOCKS WERE OVERLAPPING; AND WHEN THEY BECAME CLR; LTT X HAD NOT VACATED 7000'. SEP BTWN THE 2 WAS ABOUT 800' AND 1 1/4 MI. TFC WAS CALLED TO LTT X ON THE OVERTAKING MLG Y. LTT X SAID THE TFC WAS IN SIGHT AND WAS TOLD TO MAINTAIN VISUAL SEP. THE LGT GOT HIS EBND TFC IN SIGHT; WAS DSNDED FURTHER AND HANDED OFF TO THE FINAL CTLR; AS WERE MLG Y AND LTT X. WE HAD LESS THAN 1000' AND 3 MI BTWN THE 2 BEFORE LTT X HAD MLG Y (ABOVE AND AHEAD) IN SIGHT. LTT X MAINTAINED VISUAL SEP FROM MLG Y AND NO EVASIVE ACTION WAS TAKEN. THIS SITUATION OCCURRED BECAUSE I DID NOT ENSURE THAT WHAT I HEARD WAS ACTUALLY WHAT WAS SAID TO LTT X REGARDING HIS DSCNT. AFTER THE DISCUSSION THE TRNEE AND I HAD ABOUT DSCNT RATES OF DISSIMILAR TYPES OF ACFT; I DID NOT REQUIRE HIM TO RECLR MLG Y TO 8000' ONLY. I ALSO FAILED TO REQUIRE THE TRNEE TO KEEP THE DATA BLOCKS OFFSET AND READABLE.