COMMANDER 690 PLT SLOWED TAXIING FOR A JETSTREAM 32 COMING OFF THE RWY DIRECTLY AHEAD AND ONTO HIS TXWY. WHEN IT BECAME APPARENT THAT THE JS32 WAS NOT GOING TO STOP; THE AC690 ACFT ABRUPTLY STOPPED. THE TWR CTLR HAD NOT BEFORE; OR AFTER; MADE ANY ADVISORY OR GAVE INSTRUCTIONS TO EITHER ACFT.

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WE LANDED ON RWY 12 AND WERE SWITCHED TO THE LCL CTLR TO GND ON 121.9; AND INSTRUCTED TO USE TXWY Z. COMING UP TXWY Z; WE OBSERVED A J41 TURN L OFF OF TXWY Y ONTO EITHER TXWY A OR TXWY B (ARPT WAS ALSO LNDG RWY 19R) WITHOUT HEARING ANY COM BTWN THAT ACFT AND GND. DULLES FREQUENTLY SPLITS THE ARPT WITH E SIDE GND ON 121.9 AND W SIDE GND ON 132.45. SOMETIMES THEY SIMULCAST; OTHER TIMES NOT. WE SHOULD HAVE REALIZED WHAT WAS GOING ON AND SWITCHED FREQS SO WE COULD HEAR WHAT WAS BEING SAID TO THE ACFT TURING OFF RWY 19R; BUT WENT INTO 'SHEEP' MODE AND WAS DOING ONLY WHAT WE WERE TOLD. AS WE GOT CLOSER TO TXWYS A AND B; WE OBSERVED A J32 SBOUND ON TXWY Y; HAVING EXITED RWY 19R AFTER LNDG. AS HE APCHED TXWY A (OR TXWY B -- THEY'RE VERY CLOSE TOGETHER AND I DO NOT RECALL WHICH ONE HE WAS TURNING ONTO) HE SLOWED TO MAKE THE TURN AND WE FOOLISHLY ASSUMED THAT HE HAD BEEN INSTRUCTED TO PASS BEHIND US (THE LOGICAL SEQUENCE). APPARENTLY HE HAD NOT BEEN GIVEN ANY INSTRUCTION OTHER THAN 'TAXI TO THE GATE.' WE BEGAN SLOWING IN ANTICIPATION OF A MIXUP WHEN HE STARTED INTO HIS TURN RATHER THAN WAITING TO LET US PASS BEFORE BEGINNING SAID TURN. AT THIS POINT WE TURNED ON OUR TAXI LIGHTS AND SEVERAL SECONDS LATER WE BOTH DECIDED AT THE SAME TIME THAT THE OTHER WAS NOT GOING TO STOP AND LOCKED THEM UP. I BELIEVE THAT WAS PROBABLY THE FIRST TIME THEY SAW US; THOUGH THEY MAY HAVE BEEN HAVING THE SAME THOUGHT PROCESS WE WERE. WE WAIVED THEM ON; NOTHING WAS SAID ON OUR FREQ ABOUT IT. AFTER PUTTING THE AIRPLANE AWAY; I CALLED THE TWR AND THE SUPVR WHO HAD JUST COME ON DIDN'T KNOW ANYTHING ABOUT IT. THE WHOLE THING COULD HAVE BEEN AVOIDED BY EITHER OUR SWITCHING FREQS; OR; MORE CORRECTLY; BY DULLES LEAVING THE ARPT SPLIT ALL THE TIME. THEY CAN SIMULCAST IF THEY WANT TO DURING THE NON-BUSY PERIODS; BUT EVERYONE IN CLOSE PROX TO OTHER AIRPLANES; NEEDS TO BE ABLE TO HEAR THOSE AIRPLANES AND HEAR WHAT IS BEING SAID TO THOSE AIRPLANES. THE OTHER ISSUE HERE IS THAT I DON'T THINK ANYBODY IN THE TWR WHO WAS ACTIVELY MOVING AIRPLANES AT THE TIME KNEW THERE WAS A CONFLICT EITHER.