ATCT CTLR AT STL DECOMBINED LOCALS THEN ATTEMPTS TO CLEAR B737 HOLDING IN POS FOR TKOF. THE ACFT WAS ON THE OTHER LOCAL FREQ AND DID NOT RESPOND RESULTING IN A GAR.

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WE WERE CLRED ON TO HOLD ON STL RWY 12R. I READ BACK THE CLRNC TO TWR CLRED FINAL AND FINISHED THE PRE-TAKEOFF CHKLIST. THE CAPT TAXIED US ONTO THE RWY; SET THE PARKING BRAKE AND GAVE ME THE ACFT. TWR THEN CALLED US AND TOLD US TO TAXI OFF THE RWY AT MIKE FOR LNDG TFC AND DIRECTED ACR X TO GAR. TWR THEN INFORMED US THAT WE NEEDED TO BE ON S TWR FREQ 118.5 INSTEAD OF 120.05. WE SWITCHED TO 118.5 WHILE CLRING AT MIKE; TAXIED BACK AROUND ON ALPHA AND WERE CLRED FOR TAKEOFF. THE REMAINDER OF THE FLT WAS UNEVENTFUL. DURING CRUISE THE CAPT AND I DISCUSSED THE EVENTS AT STL. BOTH OF US WERE SURE WE HAD BEEN CLRED ON TO THE RWY AND HAD HEARD OTHER CALLS BY TWR ON 120.05 REFERENCING RWY 12R. THE ONLY EXPLANATION I COULD COME UP WITH WAS THAT I HAD RESPONDED TO A RADIO CALL FOR RWY 12L. AFTER LNDG AT TPA; THE CAPT CALLED STL TWR TO FIND OUT WHAT HAPPENED. THEY EXPLAINED THAT ONE CTLR WAS WORKING BOTH FREQS AND HAD CLRED US ON TO HOLD. THEY THEN SPLIT THE RWYS BETWEEN TWO CTLRS AND THE NEW CTLR CLRED US FOR TAKEOFF ON 118.5 WHILE WE WERE ON 120.5 WHILE WE WERE ON 120.05. WHEN WE DIDN'T RESPOND AND TAKE OFF; HE LOST SEPARATION FOR ACR X TO LAND AND HAD TO SEND THEM AROUND. TWR THEN CONTACTED US ON 120.05 TO CLR THE RWY. SINCE WE WERE CLRED ONTO THE RWY THEY DID NOT FEEL THAT IT WAS A RWY INCURSION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID MOST OF THE COMMERCIAL AIRPORT LAYOUT CHARTS AT AIRPORTS WITH DUAL LOCAL CONTROL FREQS; DO NOT CLEARLY INDICATE WHERE A PARTICULAR FREQ IS TO BE USED. HE SAID N AND S; OR NE AND SW; CAN BE CONFUSING.