LBB CTLR WORKING COMBINED POS EXPERIENCED OPERROR WHEN ISSUING A TKOF CLRNC WITH ACFT TOUCHING DOWN ON SAME RWY.

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I WAS WORKING LCL; GND; CLRNC DELIVERY COMBINED. ACFT X CALLED INBOUND AND I CLRED HIM TO LAND. MINS LATER; ACR Y (AN E145) CALLED FOR TAXI AND I INSTRUCTED HIM TO TAXI TO RWY 17R. THEN ACFT Z (THE C208) CALLED FOR CLRNC. I ISSUED CLRNC AND HE READ IT BACK. HE THEN SAID HE WAS READY FOR TAXI AND I ISSUED TAXI INSTRUCTIONS TO RWY 17R AT TXWY S. ACFT Z CALLED READY AT RWY 17R. I SCANNED THE RWY AND NOTICED THAT ACR Y WAS PROCEEDING AS INSTRUCTED AND CLR OF RWY 17R. I CLRED ACFT Z FOR TKOF ON RWY 17R FROM TXWY S. A FEW SECONDS AFTER I CLRED ACFT Z FOR TKOF; ACFT X KEYED UP AND SAID 'ACFT X IS TOUCHING DOWN.' I LOOKED OUT AND REALIZED I HAD FORGOTTEN ABOUT ACFT X AND IMMEDIATELY TOLD ACFT Z TO HOLD SHORT. ACFT Z HAD CROSSED THE HOLD LINE BUT HALTED THE ACFT SHORT OF A COLLISION PATH WITH ACFT X. I FORGOT ABOUT ACFT X WHILE ISSUING CLRNCS AND TAXI INSTRUCTIONS TO OTHER ACFT. FOR SOME REASON; IN MY MIND; HE HAD ALREADY LANDED AND CLRED THE RWY. LBB IS SO SHORT STAFFED THAT WE CONSTANTLY WORK EVERYTHING; RADAR INCLUDED; COMBINED IN THE TWR. I WAS WORKING IN JUST SUCH A SITUATION ON THIS MORNING. THE POS THAT WE WORK THIS FROM IS ON THE E SIDE OF THE TWR. THE MAIN ACTIVITY ON THIS ARPT IS ON THE W SIDE. WORKING LCL CTL FROM THE E SIDE OF THE TWR CAB IS NOT A GOOD VIEWPOINT AS YOU HAVE TO SCAN AROUND THE RADAR CTLR AND EQUIP FROM THE OTHER SIDE OF THE CAB FROM WHERE MOST OF THE ACTION IS TAKING PLACE. WORKING ALL OF THESE POS COMBINED IS NOT ADVISABLE DURING DAYLIGHT HRS; BUT IS ESPECIALLY DANGEROUS WHEN IT IS DONE FROM A VIEWPOINT THAT IS OTHER THAN OPTIMAL. FAA MGMNT LEFT THE CIC NO CHOICE; HOWEVER; BECAUSE HE AND I WERE THE ONLY CTLRS ON DUTY.