A NEAR COLLISION IS CAUSED BY CTLR GIVING INCORRECT INFORMATION AND PLT NOT MAINTAINING VISUAL SEPARATION.

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ACR X WAS ON AN IFR FLT PLAN FROM MSP-SEA. ACFT WAS WDB. I WAS THE PNF ON THIS LEG; DOING COPLT AND NONPLT DUTIES (SUCH AS TALKING ON THE RADIO). WE WERE GIVEN A CLRNC TO INTERCEPT LOC 16R; CROSS ANVIL (11.0 DME) AT 3000' MSL AND CLRED FOR THE ILS 16R APCH. THEN WERE TOLD TO MAINTAIN 170 KTS TILL PARKK (OM) AND TO CALL THE TWR AT PARKK. SHORTLY AFTER THAT WE BROKE OUT OF THE CLOUDS INTO THE CLR WITH A LOWER SCATTERED LAYER OF CLOUDS. IT WAS CLR OVER THE ELLIOT BAY AND OFF TO THE W. WE WERE GIVEN LGT TFC OVER THE BAY; WHICH WE SAW AND ACKNOWLEDGED. (THIS WAS LGT Y WHO WAS ON A VISUAL APCH TO 16 R WHO; ALSO WAS ON ANOTHER FREQ.) ABOUT THIS TIME THE APCH CTLR ADVISED US TO PLAN A SIDESTEP TO RWY 16L. AFTER THIS I REMEMBERED SEEING LGT Y EBND IN A RIGHT TURN; TURNING BASE (OUT OF OUR 2:30 POS) AND CONSIDERED HIM NO FACTOR SINCE WE WERE SO FAR AHEAD OF HIM ON THE APCH. WE HAD NO MORE COM WITH THE APCH CTLR. ALL THIS TIME WE ASSUMED LGT Y WAS BEHIND US. JUST A LITTLE BIT FROM PARKK OM; MAYBE A COUPLE OF MI; WE HAD OUR FIRST FULL UNOBSTRUCTED VIEW OF THE ARPT DUE TO CLOUD PASSING BY FLOATING EWARD BTWN US AND THE ARPT. ABOUT 1 MI FROM PARKK; I CALLED SEA TWR. IMMEDIATELY BEFORE I CALLED THE TWR I SAW LGT Y EMERGE FROM OVERHEAD MY FORWARD WINDSHIELD. MY ESTIMATE IS THAT LGT Y WAS 200-300' VERT ABOVE US AND VERY LITTLE HORIZ SEP. IN MY OPINION; HAD WE BEEN AT THE SAME ALT AT THAT POINT WE WOULD HAVE COLLIDED. THIS PROX TO THIS ACFT IS CONSIDERED BY THIS CREW MEMBER TO BE EXTREMELY HAZARDOUS AND UNACCEPTABLE. AFTER CALLING THE TFC TO MY CREW; THE CAPT STARTED AN IMMEDIATE LEFT TURN AND MORE OF A DSCNT TO GET MORE SPACING FROM LGT Y AND TO FOLLOW DIRECTIONS TO LAND ON RWY 16L. UPON PASSING US; LGT Y STILL HAD ITS GEAR UP AND CONTINUED TO PULL AWAY FROM US. WE MADE PARALLEL LNDGS; US ON RWY 16L AND LGT Y ON RWY 16R. WE ALSO NOTED HIS GEAR WAS EXTENDED AT A VERY LOW ALT ON APCH. IN MY OPINION; LGT Y DID NOT KEEP SIGHT OF US AS SHE WAS INSTRUCTED TO DO UPON RECEIVING HIS VISUAL APCH CLRNC TO RWY 16R. WE NEVER ACCEPTED A VISUAL AND COULD NOT BEGIN A SIDESTEP TO RWY 16L UNTIL IN VICINITY OF PARKK DUE TO OBSTRUCTION OT VISION (CLOUDS). BESIDES; WE WERE NEVER GIVEN CLRNC TO SIDESTEP TO RWY 26L FROM APCH--JUST TO PLAN ON IT. NOT ONE TIME DID OUR APCH CTLR MENTION THE CLOSURE RATE OF LGT Y OR ADVISE US OF HIS PROX TO OUR FLT (OTHER THAN THE ONE ADVISORY WHEN LGT Y WAS OVER ELLIOT BAY). IF PARALLEL APCHES ARE TO BE MADE IN MARGINAL WX CONDITIONS THEY SHOULD BE CLOSELY MONITORED AND MAYBE BY 1 FINAL CTLR INSTEAD OF 2 DIFFERENT CTLRS. SUPPLEMENTAL INFO FROM ACN 87135: WE WERE IN LGT Y CLRED FOR A BAY VISUAL APCH TO RWY 16R AT SEA. THE APCH CTLR TOLD US THERE WAS A WDB GOING TO RWY 26L AND TO KEEP HIM IN SIGHT. WE RPTED WDB X IN SIGHT BEFORE WE TURNED RIGHT BASE FOR RWY 16R AND KEPT HIM IN SIGHT. WE LINED UP ON RWY 16R WITH WDB X LOOKING LIKE HE WAS LINED UP ON THE LEFT AND BELOW US. HE SLOWLY SEEMED TO BE DRIFTING TO THE RIGHT AND WE KEPT HIM IN SIGHT THINKING HE WOULD CORRECT. NEAR PARKK OM HE SEEMED TO BE LINING UP ON RWY 16R; THEN QUICKLY CORRECTED.