S56 CTLR EXPERIENCED OPERROR AT 11000 FT WHEN DEVELOPMENTAL CTLR ISSUED INCORRECT ALT AND AN OVERTAKE SITUATION OCCURRED.

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I WAS TRAINING A DEVELOPMENTAL ON THE 'BEAR' SECTOR DURING OUR HVY WEEKNIGHT 'CARGO' BANK. AS NORMAL; THERE WAS A GOOD MIX OF SMALL; SLOW; CARGO ACFT MIXED WITH JETS AND TURBOPROPS. WE WERE SEQUENCING 3 STREAMS OF ARRS FROM THE N INTO A 'STACKED' DOWNWIND TO SLC AT 11000 FT AND 12000 FT. AT THE TIME; WE HAD 8 ACFT INCLUDING 1 ENTERING OUR SECTOR AND 1 LEAVING. ACFT #2 CAME TO US AT 12000 FT AND WAS DSNDED TO 11000 FT BY THE DEVELOPMENTAL. THEN ACFT #1 CHKED ON SEVERAL MINS LATER AT 17000 FT AND THE DEVELOPMENTAL DSNDED HIM TO 11000 FT ALSO. I DID NOT HEAR THIS ALT ASSIGNMENT AND ASSUMED #1 HAD BEEN CORRECTLY DSNDED TO 12000 FT. SEVERAL MINS LATER WHEN THE ACFT WERE 4 MI APART CONVERGING SLOWLY; THE TRAINEE EXCHANGED TFC INFO BTWN THE 2 ACFT AT THE ASSUMED ALTS OF 11000 FT AND 12000 FT -- TELLING ACFT #2 THAT ACFT #1 WOULD BE OVERTAKING HIM AT 12000 FT. NEITHER THE DEVELOPMENTAL OR MYSELF NOTICED THE ALT READOUT OF #1 ALREADY LEVEL AT 11000 FT. THEN A FEW MINS LATER; I NOTICED THE ALT OF #1 SHOWING 11000 FT A FEW MI AHEAD OF #2 AT 11000 FT. I ISSUED TFC TO #2 ON #1. #2 REPLIED 'IMC.' THEN I CLBED #1 BACK UP TO 12000 FT. THE ACFT APPEARED TO BE ON DIVERGING COURSES WHEN I FIRST NOTICED THEM BOTH AT 11000 FT. THE SUPVR WITNESSED THE SITUATION FROM THE POINT WHEN I KEYED UP TO ISSUE TFC. HE ELECTED NOT TO REVIEW THE RADAR DATA AND VOICE TAPE UNTIL 30 MINS LATER WHEN THE PLT OF #1 CALLED. THE RADAR DATA SHOWED THAT THE ACFT HAD CONVERGED TO WITHIN 1.5 NM AT THE SAME ALT BEFORE ACFT #1 TURNED AWAY. NO RADAR ASSOCIATE WAS STAFFED; AS I BELIEVE IS STILL REQUIRED FOR THIS POS (ON THIS BANK) BY A LCL DIRECTIVE. I WAS HAVING TO TAKE AND MAKE COORD WITH OTHER SECTORS OFF-LINE WHILE DEVELOPMENTAL SPOKE TO ACFT. THE PLT OF #1 HEARD US TELL #2 HE WAS AT 12000 FT; KNEW THAT WAS NOT CORRECT; BUT NEVER SAID ANYTHING. DATA BLOCKS WERE JUST A BIT SMALL FOR INSTRUCTOR TO SEE WELL FROM BESIDE; AND THE DEVELOPMENTAL WAS USING 5 NM RANGE MARKS CENTERED ON ARR FIX; INSTEAD OF INSTRUCTOR'S USUAL 10 NM RANGE MARKS CENTERED ON MAIN BANK. THESE AND OTHER DIFFERENT SETTINGS WERE DISORIENTING FOR INSTRUCTOR (MYSELF). DEVELOPMENTAL WAS NOT USING FLT PROGRESS STRIPS; AS I AM VERY ACCUSTOMED TO. DEVELOPMENTAL OBVIOUSLY DID NOT READ MODE C OF #1 WHEN ISSUING TFC. BECAUSE THIS BANK WAS NOT QUITE AS HVY AS IT COULD BE; AND BECAUSE THE DEVELOPMENTAL HAD BEEN DOING VERY WELL SEPARATING ACFT; I BELIEVE WE WERE LULLED INTO A FALSE SENSE OF SECURITY.