AN MD80 FLC RECEIVED A TCASII RA 'DSND' FROM AN ACFT TURNING ONTO FINAL APCH AT SEA.

Narrative

WAS BEING GIVEN VECTORS TO VISUAL APCH TO RWY 16R FROM E ARR PROC (STADIUM). AT SAME TIME; DASH 8 WAS GIVEN 'BAY VISUAL' FROM W OF ARPT. OUR ARRS WERE NEARLY COINCIDENT OVER DOWNTOWN SEATTLE WITH MD80 JUST SLIGHTLY AHEAD. AS WE EACH MERGED OVER DOWNTOWN SEATTLE; OUR TCASII ALARM WAS ACTIVATED TO 'DSND' BOTH ACFT IN SIGHT OF EACH OTHER. TCASII INSTRUCTIONS OVERRIDDEN. TWR WANTED W ARR (THE DASH 8) TO CROSS OVER AND LAND ON RWY 16L AND MD80 TO CROSS OVER AND LAND ON RWY 16R?! THIS MANEUVERING REQUEST LEFT ROOM FOR CONCERN ON MY PART. TWR CTLR VERY BUSY AND WE HAD TO WAIT SOME TIME FOR AN OPPORTUNITY TO QUESTION AND VERIFY XOVER PLAN. WE ARE ASKING FOR TROUBLE HERE: HIGH VOLUME OF RADIO TFC; CLOSE PROX OF ACFT TO EACH OTHER; CHKLIST REQUIREMENTS; VFR TFC AT BFI ON TCASII; FREQ CHANGE (APCH-TWR); POSSIBLE TCASII ALARMS. REMEDY: THESE APCHS MUST BE STAGGERED WITH MATCHED AIRSPDS AND NO XOVERS REGARDLESS OF TYPE ACFT. IN ADDITION; THIS IS ALL BEING DONE WITHOUT REGARD TO PAX PERCEPTIONS/CONCERNS.