ACR HAS NMAC WITH ANOTHER ACR FLT ON VISUAL APCH. TCASII RA.

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MY FLT WAS VECTORED BY SEA APCH FOR A VISUAL APCH TO RWY 16L AT SEA. I WAS FLYING THE ACFT. APCH ASKED IF WE HAD THE ARPT IN SIGHT; WHICH WE DID. THEY ADVISED THAT AN ACR B FLT WAS ALSO ON A VISUAL TO RWY 16R AND THEY HAD US IN SIGHT. WE SAW THE ACR B ACFT APCHING FROM THE R AND HIGH. APCH CTL ALSO ADVISED THAT AN AIR AMBULANCE WAS ON FINAL TO RWY 32 AT BOEING FIELD. I HAD THAT ACFT IN SIGHT TO THE L AND LOW. WE WERE CLRED FOR A VISUAL APCH TO RWY 16L AND TO CROSS BOEING FIELD AT OR ABOVE 2100. WE CROSSED AT 2100 FT. BTWN BOEING FIELD AND SEATAC THE ACR B FLT PASSED US ON THE APCH. THE ACR B APPEARED TO BE DRIFTING OVER TO 16L AND DSNDING ON TOP OF US. OUR TCASII WENT OFF AND TURNED RED WITH AN RA TO DSND. THE ACR B ALT WAS 1100 ACCORDING TO THE TCASII. I MOVED OVER TO THE L OF THE CENTERLINE OF 16L AND WAS APPROX 800 AGL AND LOOKING UP AT THE ACR ACFT. THE TWR ASKED OUR ALT FROM AN ALERTER GOING OFF IN THE TWR. I TOLD THEM ABOUT THE ACR B ACFT AND THAT I HAD THE GND AND THE FIELD IN SIGHT. HAD I STARTED A GAR I WOULD HAVE HIT THE ACR B ACFT. THE ACR B SAID THEY WERE LINED UP WITH 16R. I SLOWED THE ACFT TO 138 KTS. VREF WAS 124 KTS. I LET THE ACR B GET FURTHER AHEAD OF US AND CONTINUED THE APCH TO 16L AND MADE A SAFE SUCCESSFUL LNDG ON 16L. AFTER LNDG I WAS TO CALL THE SEA TWR SUPVR. I SPOKE WITH MR X AND EXPLAINED WHAT HAD HAPPENED. HE HAD NOT SPOKEN WITH THE ACR B CAPT WHEN I CALLED.