ACR X TCASII RA TOOK EVASIVE ACTION FROM ACR Y.

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ACR X DSNDING FROM 5000 FT TO 3000 FT; CLRED FOR AN ILS RWY 12R AT STL ON OR NEAR GLIDE PATH; ON CTRLINE. APCH CTL ASKED X IF HE HAD THE HVY B747 VISUALLY. REPLIED 'AFFIRMATIVE.' APCH CTL TOLD THE X TO 'KEEP UP YOUR SPD; AND I'LL LET YOU LAND AHEAD OF THE HVY B747.' I ASSUMED THE COMMUTER WAS CLOSE IN AND GOING TO LAND ON RWY 12R AHEAD OF US. AT APPROX 4000 FT; WE RECEIVED AN ALERT ON THE TCASII; AN ACFT WAS 700 FT DIRECTLY ABOVE US AND DSNDING. AT APPROX 500 FT; WE RECEIVED RA TO 'DSND; DSND.' I CLOSED MY THROTTLES; ROLLED INTO A 30 DEG BANK TO THE R; AND INCREASED SINK RATE TO APPROX 1500 FPM. WHEN THE WARNING STOPPED; I ROLLED WINGS LEVEL; ADDED PWR; AND LEVELED OFF APPROX 3000 FT. GLANCING UP TO THE L; I SPOTTED AN ATR APPROX 300 FT ABOVE ME AND 300 FT TO MY L. THE X THEN PROCEEDED TO LAND AHEAD OF ME ON RWY 12L; I LANDED ON RWY 12R. THIS INCIDENT MAKES KEEPING THE TCASII IN TA/RA VERY IMPORTANT IN PARALLEL RWY OPS; EVEN THOUGH WE DO RECEIVE NUISANCE WARNINGS OCCASIONALLY. WITHOUT THE TCASII; THIS COULD EASILY HAVE BEEN A MIDAIR COLLISION. SUPPLEMENTAL INFO FROM ACN 314895: DUE TO RADIO TFC CONGESTION MOMENTARY DELAY IN NOTIFYING TWR OF TFC IMMEDIATELY OVERHEAD. TWR REPLY: HE HAS YOU IN SIGHT; HE'S FOR THE LDA RWY 12L. REPLIED TO TWR: WE'RE DSNDING AS WE HAVE A TCASII RA FOR TFC! TWR REPLY: ACFT IMMEDIATE L TURN INTERCEPT LDA RWY 12L! CAPT AND FE VISUALLY LOCATE COMMUTER ACFT COMING INTO VIEW ON L SIDE OF ACFT; ABOVE OUR ACFT MOVING AWAY AND AHEAD OF US. ACFT LANDED APPROX 1 1/2 MI AHEAD OF OUR ACFT ON RWY 12L; WE LANDED ON RWY 12R WITHOUT FURTHER INCIDENT. DISCONTINUE USE OF LDA DME RWY 12L AT STL AND LDA DME RWY 30L ST LOUIS. ACFT ARE NOT MAINTAINING VISUAL CONTACT WHEN MAKING PARALLEL APCHS VFR. OFTEN BEING PASSED BY COMMUTER AND GA ACFT UNDER VFR CONDITION; WHEN THEY ARE TOLD TO 'MAINTAIN VISUAL CONTACT; CLRED FOR THE APCH. CLOSER RADAR MONITOR OF ACFT BY TWR WHEN VISUAL PARALLEL RWY USE IN OP.