ACR X HAD NMAC LTSS FROM LTT Y ON MULTIPLE RWY OP INTERSECTING RWYS. SYS ERROR. LTT Y LANDED WITHOUT CLRNC. PLTDEV.

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AT APPROX XX00 GMT I WAS WORKING 2 ACFT. AN LGT WAS CLRED TO LAND ON RWY 28L AND I TAXIED AN ACR X INTO POS AND HOLD ON RWY 28C. I DID NOT CLR ACR X FOR TKOF BECAUSE I WAS ENGAGED IN A CONVERSATION WITH A PLT ON A LAND LINE ABOUT HIS DEPARTING A LCL ARPT IN REDUCED VISIBILITY CONDITIONS. I HAD LOOKED AT THE MLG AND RWY 28C TO ENSURE IT WAS FREE OF TFC PRIOR TO ISSUING THE TKOF CLRNC; BUT I DID NOT SEE THE LTT Y ON VERY SHORT FINAL FOR RWY 32. I HAD NOT COMMUNICATED WITH LTT Y AT ANY TIME PRIOR TO CLRING ACR X FOR TKOF NOR HAD I OBSERVED HIM AS AN ARR FOR RWY 32 ON THE TWR D BRITE. I WAS AWARE WE WERE GOING TO CHANGE OPS BUT MISSED THE LNDG RWY CHANGE ON LTT Y. AT XX05:54 ACR X SAID 'ACR X IS ABORTING BECAUSE OF THE ACFT ON THE RWY.' ACR X HAD STOPPED PRIOR TO THE INTXN AT LTT Y ROLLED ACROSS RWY 28C. I THEN TAXIED LTT Y TO THE RAMP AND ACR X NEEDED TO RETURN TO THE RAMP TO ALLOW HIS BRAKES TO COOL OFF. THE BIGGEST FACTOR IN THE DEV WAS THE LTT Y PLT NOT MAKING ANY XMISSIONS TO THE TWR OR RECEIVING ANY CLRNC TO LAND PRIOR TO HIS LNDG ON RWY 32. PERHAPS IF I HAD NOT HAD TO STEP AWAY FROM THE POS I WOULD HAVE SEEN THE ARTS TAG OF LTT Y. BECAUSE OF THE DISTANCE FROM THE TWR TO THE APCH END OF RWY 32 AND THE SIZE OF THE LTT I DO NOT BELIEVE I WOULD HAVE OBSERVED HIS ARR BY JUST SCANNING QUICKLY DOWN THE RWY. THE VISIBILITY WAS ALSO RESTR IN FOG AND HAZE AND WHILE THE RPTED VISIBILITY WAS 6 MI; WITH THE GLARE OF THE SUN IT WOULD BE VERY HARD TO SEE A SMALL TWIN ACFT AT A DISTANCE OF 1 3/4 MI; WHICH IS THE DISTANCE FROM THE TWR CAB TO THE APCH END OF RWY 32. SUPPLEMENTAL INFO FROM ACN 208994. I WAS PIC OF LTT Y ENRTE FROM PARKERSBURG; WV; TO PIT. WE HAD COPIED ATIS INFO OFF OF THE ATIS; WHICH TOLD US TO EXPECT AN ILS TO EITHER 28L OR 28R AT PIT. WHEN WE WERE ON A HIGH 2 MI FINAL THE APCH CTLR INSTRUCTED US TO CONTACT TWR ON 119.1. THE COPLT HAD HIS HANDS FULL WITH THE CHKLIST AND MICROPHONE SO I TUNED IN 119.1. AS THIS WAS HAPPENING I WAS WATCHING THE LGT CROSS THE THRESHOLD OF RWY 28L AND WE FINISHED THE INITIAL CHKS AND IMMEDIATELY BEGAN THE FINAL CHKS. I WATCHED WHERE THE LGT TOUCHED DOWN AND I REMEMBER BEING WORRIED ABOUT HIS WAKE TURB SO I WAS WATCHING THIS ACFT TO MY L. AS WE TOUCHED DOWN I HEARD THE TWR CTLR CLR ANOTHER ACFT FOR TKOF ON RWY 28C. I THEN NOTICED ACR X ON RWY 28C (TO MY R) FOR THE FIRST TIME. HE BEGAN TO PWR UP AND STARTED ROLLING. AS WE ROLLED THROUGH THE INTXN MY FO CALLED 'Y IS XING THE CTR!' SEVERAL TIMES. ACR X ABORTED HIS TKOF ROLL. ATC THEN ASKED 'WHO IS XING THE CTR?' ATC THEN APOLOGIZED TO THE ACR X SAYING HE HAD NO IDEA WE WERE THERE; HE HADN'T SEEN US OR KNOWN WE WERE COMING. AS WE EXITED THE RWY WE REALIZED THAT A MISTAKE HAD BEEN MADE. IT THEN (FOR THE FIRST TIME) BECAME APPARENT THAT WE HAD PROBABLY NOT BEEN CLRED TO LAND. SUPPLEMENTAL INFO FROM 209003. ACR X CLRED INTO POS TO HOLD ON RWY 28C. WE WERE CLRED FOR TKOF AFTER HOLDING FOR 20-30 SECONDS. AS WE ACCELERATED THROUGH ABOUT 80 KTS MY PERIPHERAL VISION PICKED UP A CONVERGING LTT Y LNDG ON RWY 3L. I INITIATED AN ABORT. WHILE WE WERE SLOWING; LTT Y PASSED IN FRONT OF US. WE RECEIVED CLRNC TO TURN DOWN RWY 32 AND EXIT AT 'V' TO CHK OUR BRAKE COOLING CHART. TWR HAD NOT CLRED THE LTT TO LAND AND OBVIOUSLY WAS NOT AWARE OF THE LTT POS.