SMT MADE A GO AROUND DUE TO MLG TAXIING ONTO RWY. TWR CTLR WORKING ALL TWR POSITIONS COMBINED.

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THE SMT X WAS TOLD TO MAKE A SHORT APCH TO RWY 18 BECAUSE OF TFC ON A 5 MI FINAL TO RWY 23. THE MLG Y WAS INSTRUCTED TO TAXI TO RWY 23. MY ATTN WAS DIVERTED TO THE CLRNC DELIVERY POS TO ISSUE A CLRNC. THE MLG Y STOPPED WITH THE NOSE OF THE ACFT ON THE RWY AS THE SMT X WENT AROUND. AT THE TIME I WAS WORKING GND/LCL COMBINED AT THE LCL CTL POS AND FLT DATA /CLRNC DELIVERY COMBINED AT THE CLRNC DELIVERY POS. SUPPLEMENTAL INFO FROM ACN 84478: AFTER COMPLETING ALL PRETAXI REQUIREMENTS; A TAXI REQUEST WAS ACKNOWLEDGED AND ISSUED FROM THE FBO N RAMP TO RWY 23 FOR DEP. A TAXI CHKLIST WAS STARTED AND CONTINUED UNTIL APCHING THE HOLD LINE OF THE INTERSECTING RWY 18. I SLOWED THE ACFT TO A STOP SHORT OF 18 AND ASKED MY F/O TO VERIFY 'CLRED TO CROSS 18.' HE DID THIS; AND WHILE LOOKING AT THE TAXI CHART; ASKED THE GND CTL TO DEPART FROM THE INTXN OF 18 AND 23; WHICH I OVERHEARD AND KNEW WAS NOT POSSIBLE; SINCE THE 2 RWYS DO NOT INTERSECT. WHILE HE WAS STILL ON AIR; I SAID; 'NO; WE NEED TO CROSS RWY 18 AND TURN RIGHT TO GET TO THE THRESHOLD OF RWY 23.' HE IMMEDIATELY REQUESTED DEP ON RWY 23 FROM N. THE CTLR SAID THAT THAT WAS APPROVED AND TO CROSS RWY 18; TURN RIGHT AND HOLD SHORT OF RWY 23. BOTH MYSELF AND THE F/O LOOKED DOWN THE RWY 18-36 AND PROCEEDED TO BEGIN TAXIING. AFTER ABOUT 10' OF THE ACFT PENETRATED RWY 18; OUR MECHANIC-F/A WHO OCCUPIED THE JUMPSEAT SHOUTED; 'HOLD IT; HOLD IT!' I ABRUPTLY STOPPED THE ACFT AND SAW A LIGHT TWIN IN A NEAR 90 DEG RIGHT BANK PASS LESS THAN 20' DIRECTLY IN FRONT OF THE COCKPIT ABOUT 15' IN THE AIR. I IMMEDIATELY TOLD THE CTLR AND THE RESPONSE WAS; 'IT WAS MY ERROR.' MY RESPONSE WAS; 'THAT WAS A MIGHTY BIG ONE!' WAS IT NOT OUR COMPANY POLICY TO HAVE A 3-M CREW; I WOULD NOT HAVE STOPPED. THE LIGHT TWIN WOULD HAVE STRUCK THE COCKPIT ON THE RIGHT SIDE AND; IN ALL PROBABILITY; WOULD HAVE KILLED EVERYONE IN THE FRONT SECTION OF MY ACFT AND ANY PERSONS IN THE LIGHT TWIN. ONLY THE VIGILANCE OF THE THIRD CREW MEMBER AVERTED THIS ACCIDENT. THE LIGHT TWIN HAD BEEN LNDG ON RWY 18; SAW OUR ACFT AND WENT AROUND. THE LNDG/TAXI LIGHT WAS VERY LIKELY OBSCURED BY THE GND LIGHTS AND RAPIDLY RISING TERRAIN ON THE APCH TO RWY 18. ALSO; THE 90 DEG RELATIVE MOTION TO MY ACFT GAVE THE LIGHT TWIN NO MOTION WITH WHICH TO DETECT IT AS WE STARTED TO TAXI ACROSS RWY 18. THE CTLR WAS WORKING ATIS; CLRNC DELIVERY; GND CTL; TWR LCL CTL; DEP AND APCH CTL AT THE TIME OF THE INCIDENT. THIS WOULD APPEAR TO BE AN OBVIOUS EXCESSIVE OVERLOAD ON THE CTLR'S RESPONSIBILITIES. THE ATC SYS SEEMS PREOCCUPIED WITH PLACING BLAME AT GREAT EXPENSE TO PARTICIPANTS; BUT RELUCTANT TO ALLOCATE BUDGET FUNDS TO PROVIDE A SYS WHICH FITS WITH THE EVER INCREASING DEMANDS PLACED ON IT. GET POLITICS OUT AND PROFESSIONAL ASSESSMENTS/ACTIONS INTO THE SYS.