ACR ON VISUAL APCH KEEPING VISUAL SEPARATION BUT AHEAD OF SECOND ACFT HAS TCASII RA AS SECOND ACFT PASSES OVERHEAD.

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SEATAC WX WAS VFR WITH 10 PLUS MI VISIBILITY. WE WERE CLRED FOR VISUAL APCH TO 34R WHILE ON R BASE ABOUT 3000 MSL; TO MAINTAIN VISUAL SEPARATION ON AN LGT ON L BASE AT 4900 MSL. HE WAS THEN CLRED FOR A VISUAL TO 34L TO MAINTAIN VISUAL SEPARATION FROM US. WE TURNED FINAL AHEAD OF AND BELOW HIM AND WERE STABILIZED IN LNDG CONFIGN AT FINAL APCH SPD WHEN WE CROSSED THE OM. WE ALSO HAD TFC PARALLELING OUR COURSE ON OUR R; A TFC SPOTTER ACFT FOLLOWING THE N/S HIGHWAY AT 1300 MSL. OUR TCASII GAVE US A TA AND A RA NEAR 1000 FT ABOVE TDZ; INDICATING AN INTRUDER A FEW 100 FT ABOVE AND BARELY L OF OUR FLT PATH. I WAS FLYING AND CHOSE TO STAY ON OUR APCH PATH; GIVEN THE ALTERNATIVES THAT WE KNEW ABOUT. WHEN WE SAW THE 'INTRUDER' IT WAS THE LGT FLT OVERTAKING US FROM ABOVE; DSNDING. WE SAW HIM THROUGH THE VERY TOP OF MY FORWARD WINDSHIELD FIRST. HE SCARED THE HELL OUT OF US. WE LANDED OKAY. THE PROBLEM FOR US WAS MOSTLY OVERCOMING THE SCARE WE FELT AFTER SEEING THEIR VERSION OF 'VISUAL' SEPARATION AND EXPERIENCING A RA WITH NOWHERE TO GO. ALSO; THE THOUGHT OF HAVING TO TRY A GAR FOR SOME REASON UNDER THOSE CIRCUMSTANCES IS DISCONCERTING; ALTHOUGH THE ACFT GAR CAPABILITY IS GENERALLY THE BEST. I DON'T UNDERSTAND HOW A PLT CAN MAINTAIN VISUAL SEPARATION IN AN LGT TRANSPORT WHEN THE TARGET ACFT IS BELOW YOUR COCKPIT FLOOR. THE ONLY SOLUTION I SEE IS PLT DISCIPLINE. THE CTLR WORKLOAD AND RESPONSIBILITY SEEMS MAXED OUT WITH THEIR EQUIP AND STAFFING; I HATE TO SEE THEM TAKING RESPONSIBILITY FOR SPD CTL EVERYWHERE.