DEV FROM CLRNC CLRNC LIMIT OVERSHOT - FLC DID NOT 'HOLD SHORT' BECAUSE OF A RADIO FREQ MONITORING PROB WITHIN THE CREW.

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WE CONTACTED IAD GND ON 121.9 AND RECEIVED CLRNC TO TAXI TO RWY 30. WHILE APCHING W2; 2 ACFT IN FRONT OF US WERE HOLDING SHORT OF THAT TXWY. WHEN THEY CONTINUED THEIR TAXI WITHOUT US HEARING ATC TO GIVE THEM ANY INSTRUCTIONS; WE SUPPOSED THAT ATC FORGOT TO ASSIGN US ANOTHER FREQ (132.45). SURE ENOUGH; WHEN WE MONITORED BOTH 121.9 AND 132.45; WE HEARD ATC CALLING US AND CLRED US TO CONTINUE ON W2 FOR RWY 30 (NO HOLD SHORT INSTRUCTIONS WERE ISSUED AT THAT TIME). SINCE BOTH MY FO AND MYSELF WERE NOT SURE ON WHICH FREQ TO STAY ON; WE DECIDED TO CONTINUE TO MONITOR BOTH FREQS (COM #1 121.9; #2 132.45). WE CONTINUED OUR TAXI AND TAXI CHKLIST AND SINCE XA30 IS A VERY BUSY TIME; I KEPT MY EYE OUTSIDE TO OBSERVE OTHER ACFT MOVEMENTS. SOMEWHERE ALONG OUR TAXI; WE BOTH DECIDED THAT WE ARE ON 132.45. I THEREFORE TURNED COM #1 OFF AND KEPT LISTENING ON COM #2 ONLY. SUDDENLY; MY FO ASKED ME WHETHER WE WERE SUPPOSED TO HOLD SHORT OF RWY 2. I SAID NO; BUT THEN REALIZED THAT THE RADIOS WERE NOW SWITCHED AND 132.45 WAS IN COM #1. ATC INDEED GAVE US A HOLD SHORT CLRNC; BUT I COULDN'T HEAR IT SINCE I WAS STILL MONITORING COM #2. WE JUST PASSED RWY 2 WHEN WE REALIZED THE PROB SO WE CALLED GND CTL IMMEDIATELY. WE WERE TOLD TO CONTINUE TO TAXI AND SWITCH TO TWR. TO THE BEST OF MY KNOWLEDGE; NO CONFLICT SIT OCCURRED. THIS PROB COULD HAVE BEEN AVOIDED IF THE CORRECT FREQ WOULD HAVE BEEN ASSIGNED (WHY DO THEY NEVER MENTION THE USE OF A SECOND GND FREQ ON THE ATIS IN IAD)? THE CONFUSION THAT RESULTED PREVENTED ME FROM HEARING THE HOLD SHORT CLRNC.