FO OF A TURBO-PROP COMMUTER ACFT OVERSHOT FINAL DURING A R BASE VISUAL APCH RESULTING IN A NEAR MISS WITH ANOTHER ACFT ON FINAL TO THE OTHER PARALLEL RWY.

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WE WERE ON A R DOWNWIND VISUAL FOR RWY 26R. AN MD80 WAS ON AN ILS RWY 26L. THE TWR; TRYING TO BE HELPFUL; ATTEMPTED TO SPACE US BTWN THE MD80 AND THE B737 AHEAD OF HIM. THE TWR CALLED OUR BASE TURN AT SUCH A TIME AS TO PLACE US DIRECTLY ABEAM THE MD80. AS WE TURNED FINAL; THE FO; WHO WAS FLYING; LOST SIGHT OF THE RWY. ONTARIO HAS 1 FREQ FOR BOTH ILS RWYS 26L AND 26R. AT THAT TIME THEY HAD IT SELECTED TO RWY 26L. MY FO LOOKED AT HIS ILS RWY 26R PLATE AND SAW THAT THE FREQ ON THE PLATE MATCHED WHAT WAS TUNED INTO THE NAV RADIOS AND ELECTED TO INTERCEPT WHAT HE THOUGHT WAS THE LOC COURSE RWY 26R. IN REALITY IT WAS THE RWY 26L LOC. THIS PUT US APPROX 200 FT ABOVE AND 50 FT IN FRONT OF THE MD80. I TOLD THE FO SEVERAL TIMES BEFORE AND AFTER THE INTERCEPT THAT IT WAS THE LOC RWY 26L AND TO STAY R OF COURSE TO STAY CLR OF THE MD80; BUT THIS DID NOT REGISTER UNTIL HE REGAINED SIGHT OF THE RWYS AND SAW THAT HE WAS LINED UP FOR RWY 26L. HE THEN IMMEDIATELY CORRECTED TO RWY 26R. OUR TOTAL TIME ON THE RWY 26L LOC WAS LESS THAN 20 SECONDS. TRYING TO SHOOT A VISUAL APCH IN MARGINAL CONDITIONS SHOULD HAVE BEEN AVOIDED. A GAR SHOULD HAVE BEEN INITIATED IMMEDIATELY ON THE FO LOSING THE RWY. A CLRNC TO TURN BASE SO CLOSE TO ANOTHER ACFT SHOULD HAVE BEEN REJECTED. AND MY OWN HURRY; HURRY; HURRY ATTITUDE NEEDS TO BE GOTTEN RID OF. IT IS POTENTIALLY DANGEROUS AND ALWAYS STUPID.