MULTIPLE RWY OP PARALLEL RWYS ACR X TCASII TA HAD NMAC WITH ACR Y FOR PARALLEL RWYS. VISUAL SEPARATION IN USE.

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ACR X CLRED CIVET ARR TO CROSS ARNES AT 11000 FT; THEN CLRED ILS RWY 25L. WERE IMC FROM APPROX 10000 FT TO 4000 FT AGL. ACFT SLOWED FROM 250 KTS TO 210 KTS TO 190 KTS TO 180 KTS AT ATC REQUEST. DIFFICULT TO SLOW AND MAINTAIN GS WITH TAILWIND. CALLED TWR AT OM IN VMC; INFORMED THEM OF WIND; GOT CLRNC TO LAND ON RWY 25L. OBSERVED NO TFC IN OUR AREA; TCASII WAS ON 10 MI RANGE AND LOOKING BELOW. SOME TARGETS IN VICINITY OF ARPT OBSERVED ON TCASII; BUT NONE WERE A FACTOR. AT APPROX 1300 FT AGL; A TA 'TFC; TFC' WAS HEARD. I CHKED THE TCASII/VSI AND SAW A YELLOW DOT OVERLAPPING OUR 6 O'CLOCK POS WITH A PLUS 2 ANNUNCIATED. FO AND I CHKED AREA FOR TFC; OBSERVED NONE. NO CALL FROM ATC; WE ASSUMED TCASII HAD MALFUNCTIONED. WE RECEIVED A SECOND TA FROM 4:30 POS; PLUS 2; AND PROMPTLY SCANNED THE AREA AGAIN; BUT BECOMING SUSPICIOUS AND UNSURE OF A COURSE OF ACTION. THEN THE DOT ON TCASII TURNED RED AT 3 O'CLOCK; SHOWING PLUS 1; AND FO CALLED 'TFC IN SIGHT;' AND I SAW A TWIN TURBOPROP PASS 50 FT ABOVE AND 100 FT R AND MOVING SLOWLY R. FO CALLED TWR; ADVISED RA WITH ACFT ON R AND TWR REPLIED 'HE'S VISUAL TO RWY 25R.' WE SLOWED; MOVED L; REALIGNED WITH RWY 25L FROM SLIGHTLY L OF COURSE AND LANDED. THE OTHER ACFT; ACR Y; PASSED US AND LANDED ON RWY 25R. WE DON'T KNOW WHERE HE CAME FROM; WERE NOT INFORMED OF HIS PRESENCE; PURPOSE; OR CLRNC. I THINK HE WAS CLRED VISUAL TO RWY 25R AND PROBABLY NEVER SAW US IN HIS BLIND SPOT OR HE WOULD NOT HAVE PASSED US. ATC PROCS MUST BE REVIEWED OR Y NEEDS MORE SAFETY TRAINING.