NMAC AND LESS THAN STANDARD SEPARATION BETWEEN 2 ACR ACFT ON APCH AT STL. PLT DEVIATION.

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I WAS UNDER THE SUPERVISION OF CAPT ABC RECEIVING MY INITIAL OPERATING EXPERIENCE AS CAPT ON THE MLG. WE APCHED STL FROM THE E AND WERE RADAR VECTORED DOWNWIND TO RWY 12L S OF THE FIELD. RWY WAS CHANGED TO RWY 12R PRIOR TO OUR TURNING BASE. HAD TFC AT OUR 10 O'CLOCK RPTED TO HAVE BEEN ACR Y AS WE TURNED BASE. HE APPEARED TO HAVE BEEN FAR ENOUGH AWAY TO HAVE BEEN NO FACTOR. WE WERE GIVEN A TURN TO FINAL FOR RWY 12R PUTTING ACR Y BEHIND US. WE WERE COUPLED (ON AUTOPLT) AT 3300' ASL AT APPROX 160 KTS. WE OVERSHOT THE FINAL APCH COURSE. WHILE IN A RIGHT TURN CORRECTING BACK TO CENTER LINE OF RWY 12R. CAPT ABC OBSERVED TFC OVERHEAD AND SLIGHTLY TO THE RIGHT. JUDGING THAT THE TFC WAS TOO CLOSE HE INITIATED AN IMMEDIATE LEFT DSNDING TURN. HE THEN RETURNED CONTROL OF THE ACFT TO ME. WHILE IN THE LEFT BANK; I SAW THE ACFT THROUGH THE RIGHT SIDE WINDOW ABOVE AND SLIGHTLY TO THE RIGHT. I CONTINUED THE LEFT TURN AND STOPPED THE DSCNT AT 2800'. CAPT ABC IMMEDIATELY ADVISED APCH CTL THAT WE HAD BROKEN OFF THE APCH. WE WERE VECTORED TO THE N THEN DOWNWIND FOR AN ILS APCH TO RWY 12L. WE EXECUTED THE APCH AND LANDED W/O FURTHER INCIDENCE. I SUSPECT THAT THE CTLR IN TRYING TO ACCOMMODATE BOTH US AND ACR Y ATTEMPTED TO TURN US IN FRONT OF Y ONTO RWY 12R (ASSUMING THAT Y WAS LNDG ON RWY 12L; THAT HAS NOT BEEN CONFIRMED) FOR A SIMULTANEOUS APCH. IT APPEARS THAT Y CAUGHT UP WITH US WHILE WE WERE ON OUR BASE TURN AS HE WAS STRAIGHT IN. ACR Y WAS A LOT CLOSER THAN HE APPEARED IN THE DARKNESS. IT IS MY OPINION THE HORIZ SEP HAS TO BE MAINTAINED BY APCH CTL TO THESE 2 RWYS. WE CANNOT RELY ON VISUAL SEP WITH THESE RWYS BEING AS CLOSE AS THEY ARE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STILL NOT AWARE OF ACR Y TYPE. RPTR (ACR X) HAD OVERSHOT FINALS FOR BOTH RWY 12R AND 12L. ORIGINALLY HAD BEEN TOLD RWY 12L ILS AND THEN CHANGED TO RWY 12R. AUTOPLT MAY HAVE STILL BEEN COUPLED TO RWY 12L. ACR Y ON ILS FOR RWY 12L. FLT VIS ESTIMATED AT ABOUT 3 MI WITH HAZE. DOES NOT KNOW IF ACR Y HAD THEM IN SIGHT WERE NOT ON SAME APCH CTL FREQ. PIC NOT FLYING WAS IN RIGHT SEAT AND WAS FIRST TO SEE ACR Y AND INITIATE EVASIVE ACTION. THERE WAS NO IMMEDIATE FOLLOW UP OF INCIDENT; BUT WITHIN PAST FEW DAYS THE RPTR HAS HEARD FROM UNION REP THAT FAA ATC WAS INVESTIGATING AS A PLTDEV.