DC9-30 CREW FLEW THROUGH THE LOC AFTER BEING CLRED FOR THE APCH.

Narrative

IT WAS THE FO'S LEG AND WE WERE GETTING VECTORS TO JOIN THE LOC FOR RWY 27R AT ATL. I TUNED IN THE #1 NAV TO THE LOC FREQ. I THOUGHT THE FO HAD TUNED HIS #2 NAV TO THE SAME FREQ. THE CTLR GAVE US A VECTOR TO JOIN THE LOC AND CALL THE TWR. I TOLD THE FO THAT THE COURSE WAS COMING IN AND WAS LOOKING FOR THE TWR FREQ ON THE APCH PLATE. WHEN I LOOKED AT THE NAV DISPLAY; I NOTICED THAT WE HAD FLOWN THROUGH THE LOC AND WERE STILL GOING FURTHER OFF COURSE. I TOLD THE FO TO QUICKLY TURN R AND REJOIN THE COURSE. I LOOKED DOWN AND NOTICED THAT HIS NAV WAS NOT TUNED TO THE LOC FREQ BUT THE VOR INSTEAD. LUCKILY THERE WEREN'T ANY AIRPLANES CLOSE TO US ON THE PARALLEL APCH. THERE WAS TFC BEHIND AND BELOW US THAT HAD GAINED ON US. THE TWR CTLR HAD US TURN TO A 360 DEG HDG AND MAINTAIN 5000 FT. WE WERE HANDED OFF TO THE APCH CTLR WHO VECTORED US BACK FOR THE APCH WITHOUT FURTHER INCIDENT. SINCE THE WX WAS VFR AND WE HAD OTHER TFC IN SIGHT; I WAS LAX ABOUT MAKING SURE ALL THE NAVS WERE TUNED AND IDENTED. EVEN IN VFR CONDITIONS; WE NEED TO MAKE SURE ALL THE NAV EQUIP IS PROPERLY TUNED; ESPECIALLY WHEN WE ARE BEING VECTORED ON TO A PARALLEL ILS. THIS SIT CAN ALSO BE HELPED BY NEVER MOVING MAP DISPLAYS THAT GIVE YOU BETTER SITUATIONAL AWARENESS.