A320 FLT CREW HAS TO DEVIATE FROM HDG AND ALT DURING PARALLEL APCH TO LAS TO AVOID ANOTHER ACFT.

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WE WERE CLRED FOR VISUAL APCH TO RWY 25L AT LAS AT ABOUT LARRE. AN MD80 CROSSED IN FRONT OF US AND WAS ON THE VISUAL FOR RWY 25R (RWY 25R LOC WAS OTS). WE RPTED THE MD80 IN SIGHT AND WERE TOLD TO MAINTAIN VISUAL ON HIM. VISIBILITY WAS GOOD BUT THERE WAS BLOWING DUST AT THE ARPT AND IT WAS A LITTLE HARD TO SEE RWY 25R. THE MD80 WAS 1500 FT BELOW US (TCAS VERIFIED) WHEN HE STARTED A L TURN AND STARTED DRIFTING UNDERNEATH OF US. I LOST SIGHT OF HIM AT THIS TIME; AND SO DID THE CAPT. WE NOTICED THAT THE SEPARATION WAS NOW AT 1000 FT AND DIMINISHING; SO WE NOTIFIED APCH CTL THAT WE LOST THE TFC GOING UNDERNEATH US. THE APCH CTLR ASKED THE MD80 IF THEY HAD THE AIRBUS IN SIGHT FOR RWY 25L AND THEY SAID THEY DID. SHE TOLD THE MD80 CREW TO MAINTAIN THE VISUAL ON TFC FOR RWY 25L (US). THEY SAID THEY WOULD BUT COULD NOT SEE RWY 25R. THE APCH CTLR TOLD THE MD80 CREW TO 'TRACK THE LOC FOR RWY 25L; CLRED FOR THE VISUAL RWY 25R.' THE CAPT TOLD ME TO LEVEL OFF BECAUSE TCAS SHOWED THE MD80 RIGHT UNDERNEATH US AT APPROX 700 FT LEVEL (WE WERE DSNDING ON THE APCH). THE CAPT THEN TOLD THE CTLR AGAIN THAT THE MD80 WAS UNDERNEATH US; AND SHE SAID THEY HAD US IN SIGHT; CLRED VISUAL RWY 25L. THE MD80 CREW WAS LOOKING AHEAD OF THEM AT THE NEXT AIRPLANE FOR RWY 25L. (THEY COULD NOT SEE US RIGHT ABOVE THEM). THE CAPT TOLD THE CTLR THAT THE WERE LOOKING AT THE WRONG ACFT; SO SHE BROKE THE MD80 OFF TO THE R. THEN THE CTLR ASKED THEM IF RWY 25R WAS IN SIGHT. THEY SAID YES; SO SHE RECLRED THEM FOR THE VISUAL RWY 25R. THEY ARE NOW DIRECTLY ABEAM US SO WE GET AN RA ON THEM. WE COMPLY; TELL THE CTLR AND SHE SAID THEY HAVE US IN SIGHT; CLRED VISUAL RWY 25L. I CAN SEE THE MD80 SO WE MAINTAIN SEPARATION AND LAND WITHOUT ANY FURTHER PROBS. SINCE WHEN DO AIRPLANES FOR RWY 25R GET CLRED TO TRACK THE LOC FOR RWY 25L.