ACR-SMT FAILED TO MAINTAIN VISUAL SEPARATION AND CAUSED A CONFLICT WITH ACR-MLG ON FINAL APCH COURSE.

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XING SANTA MONICA VOR AT 7000' MLG X WAS GIVEN CLRNC TO DSND TO 4000'; SLOW TO 210 KTS AND TOLD TO EXPECT RWY 24R (24L WAS CLOSED DUE TO CONSTRUCTION ON THE TXWYS). MLG X ACKNOWLEDGED AND COMPLETED THE APCH CHKLIST. AT 1A MLG X WAS GIVEN DSCNT CLRNC TO 2500' AND TOLD THE RWY WOULD NOW BE RWY 25L LOC FREQ 109.9. AT 1B SMT Y TOLD ATC THEY HAD MLG X AND THE ARPT IN SIGHT. ATC TOLD SMT Y TO FOLLOW MLG X AND MLG X WAS FOR RWY 25L AND SMT Y WAS CLRED FOR THE VISUAL APCH RWY 25R. BTWN POINTS 1A AND 2A AS THE COPLT AND THE PNF; I STARTED TO LOOK TO THE LEFT OF OUR ACFT TRYING TO SPOT SMT Y. AT 2A MLG X WAS TOLD TO TURN TO A HDG OF 120 DEGS AND TO SLOW TO 170 KTS. AT THIS POINT I TOLD THE CAPT THAT SMT Y AT 12 O'CLOCK WAS SUPPOSED TO FOLLOW US AND TO KEEP HIM IN SIGHT BECAUSE SMT Y MESSED UP. SMT Y HAD ALREADY PASSED IN FRONT OF MLG X AND COULD NOT HAVE FOLLOWED MLG X TO THE ARPT. AT 3A ATC TOLD MLG X TO TURN TO HDG 180 DEGS AND THEN CHANGED IT TO 160 DEGS. ALSO TO ADVISE THE ARPT IN SIGHT. MLG X RESPONDED HDG 160 DEGS NEGATIVE ON THE ARPT. AT THIS POINT THE CAPT TOLD ME HE LOST SIGHT OF SMT Y. I ADVISED HIM THAT I STILL HAD SMT Y IN SIGHT AND WOULD TELL ATC OF SMT Y'S MESS UP. AT 4A ATC ASKED MLG X IF WE HAD THE ARPT IN SIGHT AND THAT OUR TFC WAS SMT Z ON A STRAIGHT IN APCH XING THE OM FOR RWY 25L. I ADVISED ATC NEGATIVE ON THE ARPT AND THE TFC. I ALSO ADVISED ATC THAT I HAD SMT Y THAT WAS TO FOLLOW US IN SIGHT AT 12 O'CLOCK. AT 5A ATC TOLD MLG X TO TURN TO HDG 230 DEGS INTERCEPT THE LOC TO RWY 25L. AT 5B ATC ASKED SMT Y IF HE STILL HAD MLG X IN SIGHT. SMT Y SAID NO; THAT MLG X WAS BEHIND HIM. ATC ADVISED SMT Y THAT HE WAS TO FOLLOW MLG X. THE CAPT DECIDED NOT TO INTERCEPT THE LOC AND FLY PARALLEL. AT THIS POINT SMT Y HAD ALREADY CROSSED THE LOC AND WAS TURNING BACK TO FINAL. THE CAPT AT THIS POINT WAS CALLING FOR FLAPS AND GEAR DOWN TO AVOID OUTRUNNING SMT Y. ATC ASKED SMT Y IF HE COULD GET BEHIND MLG X OR IF HE NEEDED THE LEFT RWY. SMT Y TOLD ATC THAT HE COULD NOT GET BEHIND MLG X AND THAT HE NEEDED THE LEFT RWY (THE LOC WAS ON RWY 25L). ATC THEN TOLD SMT Y HE WAS CLRED FOR VISUAL RWY 25L. AT 7A ATC TOLD MLG X TO CHANGE RWY TO RWY 25R CLRED VISUAL APCH. I ADVISED ATC NEGATIVE ON THE ARPT AND WE WERE ABOUT TO GO IFR. MLG X WAS APCHING A LCL OBSCURATION--VIS LESS THAN 1/4 MI. ATC'S RESPONSE WAS 'DON'T WORRY; YOU SHOULD SEE THE ARPT IN A MOMENT.' AT THIS POINT THE CAPT ASKED ME IF I HAD SMT Y AND COULD HE SAFELY MAKE A MISSED APCH; I TOLD THE CAPT THAT SMT Y WAS ABOVE AND TO HIS LEFT AND THAT IF HE WANTED TO MISS APCH; HE COULD. THE CAPT DID NOT FEEL COMFORTABLE EXECUTING A MISSED WHEN HE DID NOT SEE THE TFC. WE WERE BELOW THE OBSCURATION AND IN THE CLEAR; SO THE CAPT DECIDED TO CONTINUE. MLG X WAS JUST INSIDE THE OM BEFORE WE SAW THE ARPT TO MAKE THE VISUAL APCH. AT THIS POINT OUR SPD WAS MUCH GREATER THAN THAT OF SMT Y. SMT Y LOOKED LIKE HE WAS LINED UP FOR THE LEFT AND MLG X WAS LINED UP FOR THE RIGHT. THE CAPT CALLED FOR THE BEFORE LNDG CHKLIST AND A NORMAL LNDG FOLLOWED. THE CAPT MADE A PHONE CALL TO ATC (APCH CTL) AND THEIR COMMENT WAS 'SMT Y MESSED UP.' NOTHING WAS SOLVED WITH ATC. I TOLD THE CAPT THAT THIS HAS HAPPENED BEFORE AND THAT ATC WOULD DO NOTHING TO SOLVE THE PROB WITH VISUAL APCHS AND THAT WE SHOULD FILE A NASA RPT.