DEPARTING SEA; AN A320 CREW WAS INSTRUCTED TO MAINTAIN VISUAL SEPARATION FROM A B737 TEST ACFT DEPARTING ON THE L RWY AND APPROX 2 MI AHEAD. THE SID CALLED FOR THE A320 TO TURN R AT 8 DME; BUT THE DEP CTLR GAVE THE B737 A R TURN IN FRONT OF THE A320. A TCASII RA WAS RECEIVED BY THE A320 AND THE CREW WAS TOLD BY ATC THAT THEY WERE INSTRUCTED TO MAINTAIN VISUAL SEPARATION FROM THE B737.

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WE WERE CLRED FOR TKOF ON SEA RWY 34R. A B737 DOING TOUCH AND GOES ON RWY 34L DEPARTED JUST PRIOR TO US. ATC TOLD US TO MAINTAIN VISUAL SEPARATION FROM DEPARTING ACFT. WE CONTACTED SEA DEP CTL WITH THE B737 IN SIGHT. HE WAS APPROX 2 MI AHEAD AND 2000 FT ABOVE US. WE CONTINUED OUR CLBOUT ON THE SUMMA DEP SID. JUST PRIOR TO THE FIRST TURN POINT; ATC TURNED THE B737 INTO OUR FLT PATH. WE WERE UNABLE TO MAKE OUR REQUIRED TURN AND A TCASII TA WAS ACTIVATED FOLLOWED BY TCASII RA. I INFORMED ATC THAT WE WERE MAINTAINING OUR PRESENT HEADING AND RESPONDING TO THE TFC ALERT AND ADVISORY. SEPARATION WAS APPROX 1/2 MI HORIZ AND 500 FT VERT. THE B737 WAS A GREEN ACFT DIRECT FROM THE FACTORY; MORE THAN LIKELY BEING FLOWN BY FLT TEST OR INSTRUCTOR PLTS. IT WAS DOING TOUCH AND GOES AT A VERY BUSY INTL ARPT DURING PEAK HRS. DURING THE INITIAL CLBOUT THE B737 REQUESTED A VISUAL APCH TO BFI; WHICH IS IN VERY CLOSE PROX TO SEA. I THINK THE CTLR WAS TRYING TO ACCOMMODATE THE B737; NOT REALIZING THE CLOSE PROX TO OUR ACFT. I'VE NEVER BEEN DIRECTED BY ATC TO MAINTAIN A VISUAL ON AN ACFT ON CONVERGING COURSES. I HAVE NO PROB MAINTAINING VISUAL ON DIVERGING COURSES.