AN SF340 AND A BE02 COME CLOSE TOGETHER DURING AN APCH TO ORD; IL. THE SF340 IS FLYING ILS TO RWY 27L AND THE BE02 OVERSHOT THE ILS TO RWY 27R AND CONVERGES CLOSE TO THE SF340.

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I WAS CAPT (PNF). WE WERE TRACKING INBOUND ON RWY 27L LOC TO ORD. HAD BEEN ON LOC FOR 50 MI OUT COMING FROM AZO. APPROX 15 NM FROM THE FIELD TCASII ISSUED TA. WE IMMEDIATELY SAW THE TFC. WHEN WE INITIALLY SAW THE BEECH; IT WAS OUT THE R FRONT OF OUR WINDSCREEN (2 O'CLOCK POS); COMING FROM THE N AND IN A R TURN; WHICH WOULD BE NORMAL FOR THEM TO JOIN RWY 27R LOC. WE BOTH ASSUMED THE OTHER ACFT WAS JOINING RWY 27R LOC. OUR ACFT WAS ESTABLISHED ON THE COURSE; CTRED. TCASII THEN ISSUED RA TO REDUCE DSCNT. WE HAD GOOD VISUAL ON THE TFC AND CONTINUED THE NORMAL DSCNT PROFILE. TCASII THEN ISSUED CLB RA. AT THIS POINT WE REALIZED THE BEECH 1900 WAS JOINING (APPARENTLY) RWY 27L LOC. I SUGGESTED THE FO COME L TO GIVE ROOM TO OTHER ACFT. AS WE MOVED L; WE ALSO PASSED THE BEECH AND AT THAT TIME THEY SAW US AND MOVED BACK TO THE RWY 27R LOC. AFTER WE LANDED; MY FO WALKED OVER TO TALK WITH THE OTHER CREW. THEY TOLD HIM THEY THOUGHT THEY WERE CLRED ON THE RWY 27L LOC. THEY ALSO TOLD HIM THEIR TCASII WAS MEL'ED OTS. THERE IS NO SUBSTITUTE FOR THE OLD AXIOM 'SEE AND AVOID.' SUPPLEMENTAL INFO FROM ACN 396719: APCH SAID TO EXPECT RWY 27L WHEN WE CHKED ON. THE NEXT APCH CTLR THEN GAVE US A DSCNT AND TO CALL THE FIELD IN SIGHT IF ABLE. WE CALLED THE FIELD IN SIGHT AND WERE GIVEN A VISUAL APCH CLRNC. WHILE LINING UP ON RWY 27L WE NOTICED A SAAB 340 OFF OUR L WINGTIP. APCH THEN QUESTIONED IF WE HAD RWY 27R IN SIGHT AND IF WE WERE LINED UP FOR IT. WHEN WE RESPONDED THAT WE WERE LINING UP ON RWY 27L HE TOLD US TO TURN R TO 310 DEGS AND LINE UP FOR THE VISUAL TO RWY 27R. THE SAAB HAD TO TAKE CORRECTIVE ACTION TO THE L AND THEN RELINE UP ON RWY 27L FOR THE VISUAL LNDG. AT THE TIME OUR TCASII WAS MEL'ED AND WE KEPT LOOKING FOR TFC ONLY TO OUR R WHILE KEEPING THE ARPT IN SIGHT.