SMA TAXIED ONTO RWY WITHOUT CLRNC AS SMT WAS COMPLETING HIS LNDG ROLLOUT. SMT WAS ABLE TO STOP WITHOUT ANY FURTHER INCIDENT.

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I WAS WORKING LCL CTL; WX WAS VFR; WIND CALM; TFC LIGHT. RWY 24 WAS THE ACTIVE; BUT ALL RWYS (6/24; 10/28; 15R/33L; 15L/33R) WERE AVAILABLE (THIS IS A COMMON PRACTICE). SMT Y CALLS 10 MI TO THE SE FOR L DOWNWIND FOR RWY 24. I ASK IF HE WOULD LIKE STRAIGHT IN FOR RWY 33L; WIND CALM; HE AGREES. I COORDINATE WITH GND CTL FOR RWY 33L FOR LNDG SMT Y. I CLR SMT Y TO LAND RWY 33L. GND CTL CHANGES CTLRS FOR BREAKS AND I RECONFIRM RWY 33L IS MINE FOR LNDG SMT Y. ANOTHER SMT CALLS READY FOR DEP ON RWY 24. I TAXI INTO POS AND HOLD ON RWY 24; TFC LNDG RWY 33L. AN SMA CALLS ON FINAL FOR RWY 24. I CLR THE SMA TO LAND RWY 24; CAUTION WAKE TURB DEPARTING SMT. SMT Y HAS TOUCHED DOWN ON RWY 33L AND STARTED TO SLOW DOWN AS GND CTLR YELLS THERE IS AN SMA X TAXIING ONTO RWY 33L AT TXWY C. SMT Y STOPPED ABOUT 300-500 FT FROM THE SMA X AND SAID 'TWR WE HAVE SOME CONFUSION AT THE INTXN.' I SWITCHED SMT Y TO GND AND HE WAS INSTRUCTED TO CALL THE TWR ALONG WITH SMA X. THE SUPVR TALKED WITH BOTH PLTS AND CONCLUDED THAT IT WAS A PLTDEV; AS SMA X WAS INSTRUCTED TO HOLD SHORT OF RWY 33L AND HE REPEATED THE INSTRUCTION THEN WENT ON THE RWY. BOTH PLTS AND THE SUPVR AGREED NOT TO FILE ANY PAPERWORK. SUPPLEMENTAL INFORMATION FROM ACN 259328: OUR FIRST TAXI INSTRUCTION WAS STANDARD FOR DEP ON RWY 24; I.E.; S AND C AND RWY 33 HOLD SHORT RWY 24. SECOND TAXI INSTRUCTION WAS TO HOLD SHORT RWY 33; WHICH I DID ACKNOWLEDGE. I PULLED ON THE RWY BEFORE REALIZING AND BEING INFORMED BY GND THAT AN ACFT SMT X WAS ON SHORT FINAL OR HAD INDEED TOUCHED DOWN. FORTUNATELY I DIDN'T PROCEED VERY FAR ON THE RWY AND THE ACFT COULD HAVE PASSED ME BUT WAS ABLE TO STOP BEFORE THEY REACHED MY POS. IT WAS A BAD ERROR AND COULD HAVE BEEN VERY SERIOUS. THE CAUSE OF THE PROB WAS PARTIALLY THE USE FOR THE SMT Y OF RWY 33 AS WELL AS RWY 24. ALSO THE SWITCH FROM RWY 15. I WAS CONCENTRATING ON REVIEWING AGAIN DEP PROCS AS WELL AS TALKING TO SOMEONE IN THE R SEAT AND DID NOT CONCENTRATE ENOUGH ON THE SECOND INSTRUCTION. WHAT WILL PREVENT A RECURRENCE IS STRICT ATTN TO 'FIRST THINGS FIRST' AND CONCENTRATING MORE CAREFULLY ON THE LAST INSTRUCTION FROM ATC.