NEAR CONFLICT OCCURRED WHEN PHL LCL CTLR ALLOWED AN A319 AND A B737 TO CONTINUE LNDG ON XING RWYS UNTIL BOTH ACFT WERE BELOW 300 FT ON CONVERGING COURSES. PLT RPTED AN NMAC.

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ABOUT 5 MI OUT; CLRED TO LAND RWY 27R AT PHL. ABOUT 1000 FT AGL; DASH 8 APPROX 2000 FT TO THE R AT SAME ALT; (HE WAS LNDG RWY 26. RWY 26 STAGGERED BY ABOUT 3000 FT.) THIS ACFT MINIMALLY CONTRIBUTORY. B737 WAS ON APCH AND TOLD TO FOLLOW US (CONFIRMED AT ABOUT 800 FT). AT ABOUT 300 FT AGL I (PF) STATED TO THE CAPT MY CONCERNS ABOUT THE B737 LNDG RWY 35 AND ON AN INTERSECTING COURSE. IT DIDN'T LOOK LIKE THE B737 ASSIGNED LNDG RWY 35 WOULD BE ABLE TO LAND WITHOUT POTENTIAL COLLISION. DISTURBING WAS THAT FROM AROUND 300 FT AGL AND BELOW I FELT I COULD NOT GO AROUND WITHOUT THE SIGNIFICANT RISK OF FLYING INTO THE B737 APCHING RWY 35 DUE TO PITCH; SPD AND LIMITED ACFT MANEUVERABILITY. OVER THRESHOLD; I BROUGHT THRUST TO IDLE AND PITCHED ACFT NOSE DOWN TO LAND ON INITIAL PART OF THE RWY AND OBVIOUSLY (VISUALLY) INDICATE TO THE B737 PLT THAT I WAS GOING TO COMMIT TO THE LNDG. PHL ATC TOLD THE B737 TO GO AROUND WHEN HE WAS 50-75 FT AGL. I DON'T KNOW WHY THE B737 DID NOT GO AROUND EARLIER BUT RWY 35 IS A SHORT RWY AND MAYBE HE GOT FIXATED.