C402 PLT HAD RWY INCURSION AT SAT.

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I WAS IN THE TRAIL ACFT OF A FLT OF TWO. AS THE LEAD AIRPLANE CALLED FOR TAXI; I POS MY AIRPLANE BEHIND THE LEAD ACFT. AT THAT TIME I THOUGHT I HEARD ATC GIVE THE FOLLOWING TAXI CLRNC; '...ACFT Y TAXI HOLD SHORT OF RWY 30L AT TXWY S.' LEAD STARTED TO TAXI ACROSS TXWY H; TOOK A R ON TXWY G; AND THEN A L ON TXWY S; I FOLLOWED. LEAD CONTINUED TOWARD RWY 30L ON TXWY S. AS LEAD APCHED THE HOLD SHORT LINES; I BEGAN TO WONDER IF I HAD HEARD THE CLRNC WRONG. I LOOKED DOWN BOTH ENDS OF THE RWY AND SAW NO TFC SO I FIGURED I HAD HEARD THE CLRNC WRONG AND I CONTINUED TO FOLLOW THE LEAD ACFT. HOLDING SHORT OF RWYS 12L/30R GND CALLED LEAD AND ASKED IF HE WAS HOLDING SHORT OF RWY 30L. LEAD REPLIED THAT HE WAS HOLDING SHORT OF RWY 12L. CONTRIBUTING FACTORS: EARLY IN THE MORNING; DARK; RAINING; AND MARGINAL VISIBILITY. CORRECTIVE ACTION: FOR MULTI-SHIP OPS ESTABLISH AN INTERNAL FREQ; IF THERE IS ANY SUSPICION FROM ANY OF THE FLT; ADVISE THE LEAD ACFT. ANYTIME A PLT IS ABOUT TO CROSS A RWY AND IS NOT 100% SURE; ASK ATC BEFORE XING. BECAUSE WE TAXI ACROSS THE RWYS EVERY MORNING AND EVENING; (WE) PLTS MUST BE AWARE OF COMPLACENCY. SUPPLEMENTAL INFO FROM ACN 507469: I WAS THE LEAD ACFT OF A FLT OF TWO TAXIING. I CALLED GND CTL FOR TAXI AND THOUGHT HE TOLD ME TO HOLD SHORT OF RWY 12L AT TXWY S. I TAXIED ACROSS RWY 30L AND HELD SHORT RWY 12L. AFTER A FEW SECONDS GND ASKED ME IF I WAS HOLDING SHORT RWY 30L; I ADVISED HIM I THOUGHT HE SAID HOLD SHORT RWY 12L. HE SAID 'NEGATIVE; I SAID HOLD SHORT RWY 30L; YOU HAVE CROSSED AN ACTIVE RWY.' THEN I WAS TOLD TO CALL THE TWR. I BELIEVE A MISCOM BTWN ME AND GND CTL AND COMPOUNDED BY THE DECREASING WX AS WE STARTED TO TAXI WERE THE CAUSES OF THE INCURSION. I BELIEVE I SHOULD HAVE ASKED FOR CLARIFICATION OF THE TAXI INSTRUCTIONS AND A MORE ALERTNESS OF SECOND ACFT WOULD HAVE PREVENTED THE INCURSION.