AN A320 CREW; ON FINAL TO ORD; LINED UP ON THE WRONG PARALLEL RWY. WHEN QUERIED BY TWR; A CORRECTION WAS MADE; FOLLOWED BY AN UNEVENTFUL LNDG ON THEIR ASSIGNED RWY.

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ON N DOWNWIND FOR RWY 27R; ORD; ATC DIRECTS TURN TO 180 DEGS; SLOW TO 180 KIAS; DSND TO 3000 FT; RPT FIELD IN SIGHT. CAPT; HAND-FLYING; CALLS FOR GEAR DOWN; FLAPS 2 DEGS; RPT THE FIELD IN SIGHT. WE'RE IN THE TURN; I PUT GEAR DOWN; FLAPS 1 DEG AS SPD DID NOT PERMIT FLAPS 2 DEGS YET. HE SAYS HE HAS FIELD; I SAY I DO NOT. ATC SAYS INTERCEPT RWY 27R LOC. I RPT FIELD IN SIGHT; CAPT IS FLYING VISUALLY; AND ATC SAYS '180 KTS 'TIL THE MARKER; CALL TWR.' I ROGER AND CONTINUE SOME BUTTON SELECTIONS 'INSIDE' AND STILL CAN'T SEE THE RWY. I DO CALL 'LOC ACTIVE' TO CAPT AND GIVE HIM FLAPS 2 DEGS AS WE'VE SLAVED SUFFICIENTLY. I NOW LOOK OUT AND SEE RWY LIGHTS AHEAD; JUST AS ATC CALLS TO SAG WE'RE LINING UP ON THE L RWY; NOT THE R AS ASSIGNED. WE BOTH THEN PICK OUT THE RWY 27R LIGHTS AND THE CAPT CORRECTS BACK TO RWY 27R FINAL. NO CONFLICT OCCURRED; NO EVASIVE ACTION OR BREAKOUTS; BUT; HERE WAS THE POTENTIAL. WE LANDED UNEVENTFULLY. HOW PROB AROSE: AS PNF; I WAS BUSY WITH MULTIPLE REQUESTS FROM INSIDE AND OUTSIDE THE AIRPLANE; DID NOT HAVE THE FIELD IN SIGHT; BUT ASSUMED TOO MUCH WHEN THE CAPT SAID HE HAD 'RWY IN SIGHT.' HE WAS HAND-FLYING WITH FLT DIRECTORS OFF; VISUALLY MANEUVERING AND DIDN'T CATCH THE 'LOC' WHEN I CALLED IT. I WENT BACK INSIDE TO COMPLETE TASKS AND DIDN'T NOTICE HIS OVERSHOOT. A N XWIND MAY HAVE CAUSED HIM TO MISJUDGE THE TURN ON AND HE MAY HAVE LOCKED VISUALLY ON THE FIRST RWY APCH LIGHTING HE SAW. CONTRIBUTING FACTORS: PF TURNED OFF BOTH FLT DIRECTORS WHEN I WAS 'HEADS DOWN' WITHOUT TELLING ME (WE FLY WITH BOTH 'ON' OR BOTH 'OFF' VISUALLY). THIS TOOK AWAY A COMMON CUE WE LOOK FOR IN INST GUIDANCE. I WAS DISTRACTED FROM MONITORING HIS VISUAL PROGRESS AND THE INST GUIDANCE AS I PERFORMED OTHER TASKS AND WAS WATCHING FOR SPD TO DISSIPATE ENOUGH FOR MORE FLAPS. USUALLY WE FLY 'COUPLED' AT THESE BUSY PLACES WITH FMS ARMED TO INTERCEPT AN ILS; ALLOWING LESS CHANCE OF ERROR WHEN ONE IS TASK SATURATED. IT SEEMS 'HAND-FLYING' EFIS BIRDS ON VISUAL APCHS REQUIRES MORE VIGILANCE ON THE PART OF THE PNF TO MONITOR FLT PATH FIRST.