NMAC CAUSED BY AN MD88 CAPT OVERSHOOTING FINAL WHEN HE INADVERTENTLY TURNED TOWARD THE WRONG R PARALLEL RWY AFTER BEING INSTRUCTED TO CHANGE LNDG RWY TO THE R PARALLEL RWY OF THE SET OF PARALLEL RWYS TO WHICH HE HAD ORIGINALLY BEEN CLRED AT PHX.

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DURING APCH TO PHX WE WERE SET UP FOR RWY 25L; ILS; BACKING UP THIS VISUAL APCH. TWR THEN CLRED US FOR A VISUAL APCH TO RWY 25R WHICH WE ACCEPTED. THE CAPT TOLD ME TO SET UP THE FMS FOR RWY 25R AS HE MANEUVERED TO LAND. AS I LOOKED UP AFTER PROGRAMMING THE FMS; IT APPEARED THAT THE CAPT WAS OVERSHOOTING RWY 25R SO I ASKED HIM WHERE HE WAS HEADED AND POINTED OUT HIS OVERSHOOT. HE WAS HEADED FOR RWY 26 ON THE N SIDE OF THE ARPT AND DID NOT REALIZE HE WAS GOING TO THE WRONG RWY AND DID NOT CORRECT BACK TO THE L OR S IMMEDIATELY BECAUSE HE DIDN'T THINK OR BELIEVE HE WAS GOING TO THE WRONG RWY. WE RECEIVED A TA FROM THE TCASII AND THE DISPLAY SHOWED TFC 300 FT BELOW AND SLIGHTLY TO THE R OF US. THE CAPT WAS HESITANT IN MAKING A CORRECTION; NOT REALIZING STILL THAT HE WAS IN FACT LINING UP ON RWY 26R. I THEN DEMANDED THAT HE IMMEDIATELY GET BACK TO THE L AND STAY LEVEL BECAUSE OF THE ACFT BELOW US TO THE R. HE THEN BEGAN A TURN TO THE L TOWARD RWY 25R; BUT SLOWLY DSNDED TO GET TO THE GLIDE PATH. AN RA SOUNDED WITH 'REDUCE DSCNT' WHEN I SAID AGAIN TO GO L AND LEVEL OFF. WE LANDED UNEVENTFULLY ON RWY 25R; BUT I ESTIMATE THAT WE WERE ALMOST DIRECTLY OVER THE TWIN ENG PROP ON RWY 26 WITH LESS THAN 300 FT VERT SEPARATION. IT MAY HAVE BEEN AVOIDED IF THE LOC FOR RWY 25R WAS ACTIVATED AND THE FREQ ANNOUNCED BY THE TWR WHEN CLRNC WAS GIVEN BY TWR TO SWITCH TO RWY 25R AT THE LAST MIN.