A B767 TAXIING FROM A HANGAR TO GATE YX. A B757 TAXIING FROM GATE YX TO THE HANGAR. BOTH HAVE THE SAME CALL SIGN AND BOTH MOVING AND BOTH EXECUTE A RWY INCURSION.

Narrative

I WAS INVOLVED IN AN ACFT SWAP BTWN THE TERMINAL AND THE HANGAR. I WAS TAXIING A B757; RADIO CALL SIGN ACFT 123 FROM THE TERMINAL TO THE HANGAR. AT THE SAME TIME ANOTHER COMPANY JET; A B767; CALL SIGN ACFT 123 WAS RELOCATING FROM THE HANGAR ENRTE TO THE GATE YX. AFTER PUSHING FROM GATE YX ON TO TXWY A; WE WERE CLRED TO TAXI VIA TXWYS A; D AND THEN ORDERED TO HOLD SHORT OF RWY 28L. AT THE SAME TIME OUR COMPANY JET ACFT 123 WAS APCHING RWY 28R FROM EITHER TXWY K OR TXWY E. THE TWR TOLD ACFT 123 TO HOLD SHORT OF RWY 28R. I WAS UNSURE IF THE TWR HAD GIVEN US INSTRUCTIONS TO HOLD SHORT OF RWY 28L OR RWY 28R. AS WE SLOWLY APCHED 28L I NOTICED A JET COMPLETING A LNDG RUN ON RWY 28R. SHORTLY AFTER; THE TWR ORDERED ACFT 123 TO HOLD SHORT OF RWY 28R. I ASSUMED (MISTAKENLY) THE HOLD SHORT ORDER FOR US WAS INTENDED FOR RWY 28R. SEEING NO ACFT OR INBOUND TFC ON RWY 28L; I SLOWLY PROCEEDED ACROSS RWY 28L AND STOPPED SHORT OF RWY 28R. IT WAS THEN THE TWR INFORMED US THAT WE HAD CROSSED RWY 28L WITHOUT CLRNC. THE REMAINDER OF THE TAXI TO HANGAR PROCEEDED WITHOUT INCIDENT.