WRONG RWY APCH IN A NIGHT OP.

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SIMULTANEOUS ILS APCHS TO RWY 27L&R WERE IN PROGRESS. OUR FLT WAS TURNED TO A 180 DEG HDG; AND THEN SHORTLY TO A 240 DEG HDG TO INTERCEPT THE RWY 27R ILS AND CLRED FOR THE APCH. THE ARPT WAS VISIBLE ONLY INTERMITTENTLY DUE TO A SCATTERED LAYER OF CLOUDS AT ABOUT 1500 FT AGL. APCH CTL TOLD OUR FLT TO CONTACT THE TWR AT 'RECKS' FAF. THE PF HAD A DME AND I KNEW FROM A VISUAL SIGHTING OF THE ARPT; THAT WE WOULD BE INTERCEPTING THE RWY 27R ILS SHORTLY. I ASKED PERMISSION TO JOIN THE PF ON THE ILS; AND AFTER AN 'OK;' I TUNED IN THE RWY 27R ILS. AS I DID; I NOTED AND CALLED THE GS ACTIVE. AT ABOUT THIS TIME; I SIGHTED THE RWY AND THE PF TURNED TO ALIGN WITH THE RWY. WE CONTINUED FROM 4000 FT TO ABOUT 2000 FT WHEN THE O'HARE FINAL MONITOR CAME ON TWR FREQ AND TOLD US TO TURN TO A 300 DEG HDG TO INTERCEPT THE RWY 27R ILS. WE WERE TOLD WE WERE S OF COURSE ON THE RWY 27L ILS. I SAW WE WERE ALIGNED WITH THE WRONG RWY. I HAD RWY 27R IN SIGHT; AND WE WOULD INTERCEPT THE ILS. THE MONITOR CLRED US FOR A VISUAL APCH AND LNDG ON RWY 27R. THE REMAINDER OF THE APCH AND LNDG WERE UNEVENTFUL. WHAT CAUSED THE PROB WAS THE PF TUNED IN THE ILS FREQ FROM THE FACING PAGE; RWY 27L ILS; WHILE I TUNED IN RWY 27R ILS. I BECAME OVERLY OCCUPIED LOOKING FOR THE RWY; .RATHER THAN BACKING UP THE PF WITH INFO ABOUT BEING NOT ON COURSE ON MY ILS; AND CHKING THE FREQ ON THE OTHER ILS.