FLC OF A DC9 REPLIED TO THE WRONG CALL SIGN FOR ANOTHER DC9 ON THE OPPOSITE SIDE OF THE ARPT RESULTING IN TAXIING ACROSS A RWY WITHOUT CLRNC. BOTH ACFT WERE TAXIING TO THE SAME RWY FOR TKOF; BUT COULD NOT SEE EACH OTHER SINCE THEY WERE ON THE OPPOSITE SIDES OF PARALLEL RWYS.

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ACR X WAS HOLDING SHORT OF RWY 30L AT TXWY N. WE WERE CLRED TO CROSS RWY 30L AT TXWY N FOR THE FULL LENGTH OF RWY 30R. WE READ BACK TO ATC ACR X CLRED TO CROSS RWY 30L. WE CLRED FOR TFC OUT THE R AND L WINDOWS; TURNED ON OUR ICE AND NACELLE LIGHTS AND CROSSED RWY 30L AT TXWY N. NOTHING WAS SAID TO US TO INDICATE WE HAD MISTAKEN ANY CLRNC. 2 1/2 HRS LATER; UPON OUR RETURN TO STL; WE WERE ADVISED THAT ATC HAD BELIEVED THAT WE HAD CROSSED RWY 30L AT TXWY N WITHOUT A CLRNC. WE MET WITH OUR CHIEF PLT AND HAD A CONFERENCE CALL WITH THE STL ARPT ATC FACILITY. THEY STATED THAT THEY HAD CLRED ACR Y TO CROSS RWY 30R AT TXWY N FOR RWY 30R FULL LENGTH. THEY ALSO STATED THAT ACR Y WAS BEING CTLED ON 121.90; WHILE WE WERE ON 121.65. THEY SAID THAT ONE CTLR WAS WORKING BOTH FREQS AND THAT THEY WERE IN THE PROCESS OF SPLITTING THEM. THEY SAID THAT IN REVIEWING THE TAPES; THAT THEY HEARD THE ACR Y BEING CLRED TO CROSS RWY 30R AT TXWY N FOR RWY 30R FULL LENGTH AND THAT THEY HEARD PART OF ACR Y READBACK AND SOME OTHER GARBLED TALKING. THEY WENT ON TO EXPLAIN THAT WHEN ONE CTLR IS WORKING BOTH FREQS THAT THE CTLR CAN'T HEAR BOTH FREQS AT THE SAME TIME. IT WAS ALSO MENTIONED THAT THEY WERE NEARING A SHIFT CHANGE. WE IN ACR X HONESTLY BELIEVED THAT WE WERE GIVEN OUR CLRNC TO CROSS RWY 30L AT TXWY N FOR THE FULL LENGTH RWY 30R. WE READ BACK OUR CLRNC TO ATC; HEARD NO OBJECTION; CLRED FOR TFC; AND PROCEEDED AS INSTRUCTED. NO ACFT WERE ON FINAL OR ON THE RWY TO EITHER THE R OR L. WE BELIEVE THAT THIS COULD BE PREVENTED BY HAVING THE GND CTLR WORKING ONLY ONE FREQ SO THAT THEY CAN HEAR READBACKS OR QUESTIONED WHAT THE 'GARBLED TALKING' WAS.