ACR X HAD LTSS SEPARATION FROM LNDG ACR Y WHILE XING RWY. SYS ERROR.

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ACR X CLRED TO LAND RWY 4R SHORT OF RWY 8L. AFTER LNDG RWY 4R; TOLD TO HOLD SHORT RWY 4L ON KILO TAXIWAY (NOTE: KILO TAXIWAY CROSSES BOTH RWYS 4L AND 8L AT THE SAME SPOT). HEARD TWR TELL ACR Y TO CHANGE RWY TO 4L. HVY WDB LNDG RWY 8L. WE CAME TO A STOP ON KILO TAXIWAY; SET PARK BRAKE. TWR THEN TOLD US 'CLRED TO CROSS RWY 8L; TFC CLRING UPFIELD; CONTACT GND .9.' FO REPEATED BACK CLRED TO CROSS RWY 8L. WE COULD SEE THE WDB DECELERATING ON RWY 8L AND LOOKED DOWN RWY 4L; BUT COULD NOT SEE ACR Y. ACR X TAXIED ACROSS RWY 8L. WHILE XING TWR TELLS ACR Y; SORRY ABOUT THAT; I DON'T KNOW WHY HE CROSSED (MEANING US; ACR X). FO INFORMED TWR WE WERE CLRED TO CROSS. LATER GND CTL CALLED US AND SAID TWR WOULD LIKE TO APOLOGIZE FOR THE MIX-UP. I THINK THIS OCCURRED DUE TO THE MANY NEW CTLRS AND SITTING ON KILO TAXIWAY; WE DO NOT HAVE A COMPLETE VIEW OF RWY 4L. AT THAT ANGLE IT IS IMPOSSIBLE TO SEE THE APCH END OR TOUCHDOWN ZONE. WE LOOKED CAREFULLY BEFORE XING AND SAW NO CONFLICTING TFC -- OR -- WE WOULD NOT HAVE CROSSED. ALSO; IT IS STANDARD PRACTICE THAT EVEN THOUGH TFC HAS JUST LANDED AND IS NOT ACTUALLY CLRING THE RWY; WE ARE TOLD TO CROSS -- TFC CLRING. NOT SEEING ACR Y; I ASSUMED IT TO BE EITHER STILL ON THE FINAL OR CLRING THE RWY. INSTRUCTIONS TO CROSS AT KILO ARE 99% OF THE TIME GIVEN AS JUST CLRED TO CROSS RWY 8L.