A CHALLENGER 604 FLT CREW INADVERTENTLY MISTAKE THE WRONG RWY THAT THEY ARE CLRED ACROSS. THE CTLR WAS ABLE TO ALERT THE FLT CREW BEFORE IT BECAME A HAZARD.

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ON TAXI OUT FROM THE NW RAMP (FBO) AT PDK ARPT; WE WERE CLRED TO 'TAXI FOR RWY 2R; HOLD SHORT RWY 2L.' BOTH MYSELF AND THE CAPT FELT WE HAD A GOOD UNDERSTANDING OF THE TAXI RTE; BUT WE DID NOT FORMALLY BRIEF THE RTE. WE UNDERSTOOD THAT ENRTE TO THE APCH END OF RWY 2L WE WOULD HAVE TO CROSS RWY 9/27. AS WE TURNED R OUT OF THE FBO AND TAXIED DOWN TXWY B; WE FAILED TO MAKE THE 45 DEG R TURN ONTO TXWY A AND CAME UPON RWY 2L/20R. AT THIS TIME WE WERE ENGAGED WITH OUR BEFORE TKOF CHKLIST AND MORE SPECIFICALLY WERE ENGAGED WITH OUR DEP BRIEFING. THIS WAS OBVIOUSLY DISTRACTING BECAUSE AS WE APCHED RWY 2L/20R; THE RWY WE WERE TOLD TO HOLD SHORT OF; WE BOTH ASSUMED WE WERE NATURALLY APCHING RWY 9/27 AS WE EXPECTED. WE CLRED BOTH ENDS OF THE RWY AND CONTINUED ACROSS. THE GND CTLR CAUGHT OUR ERROR AND INSTRUCTED US TO NOW HOLD SHORT OF RWY 2R/20L WHICH WE DID. THIS WAS WHEN WE REALIZED OUR ERROR. THERE WAS NO TFC CONFLICT. GND CTL REDIRECTED US WITHOUT FURTHER INCIDENT. IN MY OPINION; THE 2 FACTORS INVOLVED WERE THE DISTR OF THE TKOF BRIEF AND A CLASSIC CASE OF SEEING ONLY WHAT WE EXPECTED TO SEE. EVEN THOUGH AFTERWARD WE BOTH RECALLED READING THE RWY 2L/20R RWY SIGN; IT DID NOT REGISTER WITH US SINCE WE WERE EXPECTING TO BE XING A RWY ON THE WAY OUT. WE ALSO FEEL A CONTRIBUTING FACTOR WAS THE FACT THAT THIS WAS A DEADHEAD FLT. AS MUCH AS WE HATE TO ADMIT IT; IT SEEMS THAT THE REQUIRED LEVEL OF ATTN ON DEADHEADS IS HARDER TO MAINTAIN. TO PREVENT THIS IN THE FUTURE; I WILL ALWAYS BRIEF THE TAXI RTE EVEN AT FAMILIAR ARPTS (AS THIS WAS); AND WHENEVER POSSIBLE CONDUCT THE DEP BRIEF PRIOR TO TAXI. ALSO; PRIOR TO XING A RWY; ASIDE FROM SIMPLY CLRING BOTH ENDS; I WILL VERBALLY CALL OUT AND CONFIRM THE RWY NUMBER TO VERIFY IT IS THE RIGHT ONE.