DEVELOPMENTAL CTLR RECEIVING OJT ON THE ARR RADAR POS DSNDED THE WRONG ACFT; A B737 VERSUS A BE02; DUE TO DATA BLOCK OVERLAP. THE INSTRUCTOR CTLR WAS BRIEFING HIS RELIEF AND SAW THE POTENTIAL CONFLICT AND STOPPED THE B737'S DSCNT; BUT STANDARD SEPARATION WAS LOST. TFC VOLUME HAD BEEN MODERATE; BUT HAD JUST SLOWED DOWN AND BOTH CTLRS RELAXED THEIR ATTN TO DETAIL.

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I HAD BEEN TRAINING A DEVELOPMENTAL WHO HAD DONE A VERY GOOD JOB WITH THE INBOUND PUSH. ONLY 3 ACFT WERE ON FREQ AND WORKLOAD WAS LIGHT. 2 OF THE ACFT WERE ON CLOSE; YET DIVERGING TRACKS. I TURNED MY HEAD TO COORDINATE THE END OF OJT WITH RELIEVING SPECIALIST AND HEARD DEVELOPMENTAL ISSUE A CTL INSTRUCTION; BUT COULD NOT UNDERSTAND IT DUE TO BACKGND NOISE. I LOOKED CLOSELY TO SEE WHAT ACTION HAD BEEN TAKEN AND OBSERVED THE HIGHER OF THE 2 CLOSELY SPACED ACFT BEGIN TO DSND. FEELING THAT THIS SPACING (ALTHOUGH PROBABLY LEGAL) WAS NOT PARTICULARLY CONDUCIVE TO SAFETY; I ISSUED EVASIVE TURNS AND STOPPED THE DSNDING ACFT. AFTER RE-ESTABLISHING WHAT I FELT WAS POSITIVE SEPARATION; I QUERIED THE PLT IF HE HAD RECEIVED A TCASII RA. HE REPLIED THAT THEY HAD RECEIVED A TA. HUMAN PERFORMANCE FACTORS WERE KEY HERE. REGARDLESS OF HOW WELL A DEVELOPMENTAL MAY PERFORM; INSTRUCTORS CAN NEVER LET THEIR COMFORT LEVEL REPLACE THEIR CAREFUL SURVEILLANCE OF THE POS. ALSO; THIS WAS A CLASSIC OPERROR SIT IN THAT THE WORK WAS DONE AND COMPLEXITY MINIMAL LEADING TO A RELAXATION OF ATTN TO DETAIL. SUPPLEMENTAL INFO FROM ACN 368389: A B737 6000 FT ON A DOWNWIND. BE02 5000 FT ON A DOGLEG BELOW THE B737. I GAVE THE B737 DSCNT INSTRUCTIONS INSTEAD OF THE BE02. ARTS TAGS WERE ON TOP OF EACH OTHER AND THE NUMBERS WERE TRANSPOSED. THE B737 DSNDED TO 5500 FT WHEN THE INSTRUCTOR CAUGHT THE ERROR AND GAVE FURTHER DIVERGING COURSE. TFC WAS CALLED TO BOTH ACFT; AND THEY BOTH SAW EACH OTHER. THIS IS MY FIRST RADAR POS. I HAD APPROX 14 HRS OF OJT AT THIS POINT. THE EVENT OCCURRED DURING A MODERATE TO HVY ARR PUSH. I WAS WORKING APPROX 8-10 ACFT.