SMA STARTED AND COMPLETED TKOF WHILE ANOTHER SMA WAS STILL ON THE RWY AFTER LNDG.

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I WAS PERFORMING DUTIES AT THE LC-1 (LCL CTL) POS WHEN SMA X LANDED ON RWY 18R. HE SLOWED AFTER LNDG AND I INSTRUCTED HIM TO TURN R AND CONTACT GND CTL. (HE WAS APCHING A TXWY AHEAD AND TO HIS R.) I THEN INSTRUCTED SMA Y; WHO HAD BEEN HOLDING SHORT OF THE RWY THAT HE WAS CLRED FOR TKOF. I ANTICIPATED THAT SMA X WOULD MAKE THE TURN AND HOW HE MISSED IT I'LL NEVER KNOW BUT I NOTICED HE HAD BYPASSED THE TURN OFF. SMA Y HAD JUST STARTED HIS DEP ROLL AND I BROADCAST TO HIM TO CANCEL TKOF CLRNC. HE REPLIED IT WAS TOO LATE AND DEPARTED WHILE SMA X WAS STILL ON THE RWY. SMA X WAS WELL DOWN THE RWY (I ESTIMATED 4000 FT WHEN SMA Y ROTATED). IT WAS DARK AND DOWN THE RWY (I ESTIMATE 4000 FT WHEN SMA Y ROTATED). IT WAS DARK AND I JUST LOST THE SMA X IN THE RWY LIGHTS. I COULD MAKE RECOMMENDATIONS ALL DAY LONG AS TO HOW MORE STAFFING IN THE TWR IN THE COORDINATOR POS MIGHT HAVE HELPED OR AS TO HOW A TWR CAB SUPVR IN THE TWR MIGHT HAVE BEEN THAT OTHER SET OF EYES THAT COULD HAVE HELPED TO AVERT THIS OCCURRENCE BUT I CAN ONLY ATTRIBUTE WHAT HAPPENED TO HUMAN ERROR. NEITHER THE CAB COORDINATOR NOR THE CAB SUPVR POS WERE STAFFED BUT TFC; WHILE STEADY; WAS NOT WHAT I WOULD CALL PEAK TFC AND DURING THIS TIME OF DAY THESE POS ARE NOT NORMALLY STAFFED.