SEPARATION OF TFC DURING VISUAL APCH.

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ATIS WAS -X 11 SCATTERED 3HK WIND 1005. WE WERE ASKED TO RPT VISUAL WITH A B767; 12 O'CLOCK; 6 MI FOR VISUAL TO RWY 25L. WE RPTED THAT ACFT BUT COMMENTED THAT WE WOULD LOSE VISUAL WHEN WE ENTERED THE LOWER VISIBILITY ON APCH. APCH CTL SAID TO ADVISE WHEN WE LOST VISUAL; AND CLRED FOR A VISUAL RWY 25L BEHIND THAT ACFT. LATER; APCH CTL ADVISED TFC ON OUR L AND SLIGHTLY BEHIND; FOR RWY 25R. SUBSEQUENTLY; THAT ACFT PASSED BEHIND US AT VERY CLOSE RANGE AND VERY NEAR OUR ALT. WE RECEIVED TCASII TA AND RA ON THAT ACFT. WE ADVISED APCH CTL OF THIS AND WERE ADVISED TO 'TURN THAT MACHINE OFF; WE HAVE A LOT OF AIRPLANES OUT THERE.' THE ACFT APPEARED; NEXT; PARALLEL; ON OUR R SIDE; APPROX 100 YARDS AWAY. SUBSEQUENTLY; WE LOST VISUAL WITH OUR LEAD ACFT (B767) AND ADVISED THE TWR. TWR REPLIED; 'PRESS ON YOU WILL SEE THE ARPT; CLRED TO LAND RWY 25L.' SUMMARY: THE PARALLEL ACFT WAS TOO CLOSE FOR COMFORT. WE WERE VISUAL WITH NO VISUAL ON LEAD ACFT OR ARPT. WE WERE TOLD TO TURN THE TCASII OFF. QUESTION: WHO WAS VFR AND WHO WAS IFR AND WHAT LEGAL SEPARATION WAS BEING USED? STATEMENT: THIS PRACTICE IS EXTREMELY DISTRACTING TO THE PLTS; NOT TO MENTION AN UNSAFE PRACTICE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR WAS UNCERTAIN HOW A VISUAL APCH WORKS. HIS FEELING WAS THAT IF VISUAL SEPARATION IS LOST MOMENTARILY THEN IFR SEPARATION SHOULD BE IMMEDIATELY IMPOSED. HIS CONCERN WAS THAT HE COULD NOT SEE THE TFC THAT WOULD BE PARALLEL TO HIM THROUGHOUT THE APCH. YET; HE DID REALIZE THAT A SMALLER ACFT WOULD FLY BEHIND HIM TO LINE UP ON THE R RWY SINCE HE WAS LNDG ON THE L RWY. DURING THE TIME THE APCHING ACFT MOVED FROM L TO R IT WAS IN CLOSE PROX; THUS IT SET OFF TA AND RA ON THE TCASII. WHEN THE FLC TOLD THE APCH CTLR OF THE RA; THE CTLR SAID; 'TURN OFF THAT MACHINE.' THIS IS WHAT CONCERNED THE FLC GREATLY. IT BOTHERED THEM THAT THEY WERE TOLD TO TURN OFF THE TCASII. SUPPLEMENTAL INFO FROM ACN 290104' I SAW IN MY SCAN OF RPTED TFC; THE ACFT OFF TO THE L APPROX 4 MI A LITTLE HIGH. NEXT THING TO MY AWARENESS WAS A SERIES OF AUDIBLE AND INST WARNINGS OF AN ACFT IN OUR PROX WHICH WERE CHANGING RAPIDLY IN A CONFUSION OF COMMANDS. THE FO LOOKING TO THE R SAID; 'HE'S RIGHT OUT HERE.' I DID NOT TRY TO LOOK SINCE THE APCH WAS TOO FAR ALONG.