LESS THAN STANDARD SEPARATION BETWEEN COMMUTER AND SMA. OPERATIONAL ERROR.

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FULL PERFORMANCE CTLR WAS CONDUCTING TRNING OF DEVELOPMENTAL ON A COMBINED POS. WHEN TFC REQUIRED POS TO BE DECOMBINED; ANOTHER CTLR CAME IN TO ASSUME RESPONSIBILITY OF POS. FPL AND DEVELOPMENTAL HAD TO OPEN ANOTHER POS ONE RADAR SCOPE DOWN (NEXT POS). FPL UNPLUGGED AND RELIEVING CTLR PLUGGED IN POS WITH DEVELOPMENTAL WHILE FPL OPENED UP NEW POS. DEVELOPMENTAL FORGOT OVERFLT Y AT 5000' AND DSNDED X FROM 6000 TO 4000'. FPL THAT WAS TRNING BECAME AWARE WHEN SHE OBSERVED AND HEARD DEVELOPMENTAL TURNING COMMUTER X OUT OF THE WAY. AT THIS POINT ACFT HAD LESS THAN STANDARD SEPARATION. RELIEVING CTLR DID NOT TAKE ACTION. HE HAD NOT BEEN GIVEN A DETAILED RELIEF BRIEFING SINCE ONLY A SHORT PERIOD OF TIME WOULD HAVE ELAPSED UNTIL NEW POS WAS OPENED. MANY TIMES CTLRS WORK POSITIONS COMBINED WHEN TFC IS LIGHT. WHEN THIS IS DONE; THE CTLR WORKING MAY BE SITTING AT A RADAR SCOPE THAT ANOTHER CTLR WILL HAVE TO COME IN AND RELIEVE HIM AT BEFORE HIS SESSION IS THROUGH. THIS REQUIRES A 'MUSICAL CHAIRS' TYPE OPERATION WITH NO TIME TO GIVE A BRIEFING TO THE RELIEVING CTLR. I'VE LEARNED--WORK AT YOUR OWN SCOPE--ESPECIALLY WHEN TRNING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR GAVE YRS OF ATC FOR HERSELF AND TRNEE. TOLD OF ROLE IN INCIDENT. LOCATION AND RELATIVE POSITIONS OF ACFT. CLOSEST PROX 1 MI WHEN BOTH AT 5000'. WX WAS VMC AND BOTH ACFT WERE GIVEN TFC AND SAW EACH OTHER. NEITHER PLT FILED ANY COMPLAINT AND THERE WAS NO FORMAL RPT OF INCIDENT. C/A ACTIVATED AFTER SEP LOST. FAIRLY LONG CONVERSATION REF WHAT IS WRONG IN FAA ATC SYS.