TKOF ON OCCUPIED RWY BY SMA. UNAUTH UNCOORD RWY ENTRY BY ACR ON A SINGLE RWY OP; OPPOSITE DIRECTION TFC.

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AS CAPT OF FLT FROM BZN-BIL; I TAXIED TO RWY 30 FOR DEP. IT WAS NECESSARY FOR US TO WAIT SEVERAL MINS FOR IFR RELEASE FROM SLC CENTER DUE TO AN INBOUND OTHER AIRLINES FLT. WE WERE MONITORING BOTH ATC FSS ADVISORY FREQS TO ASCERTAIN THE POSITIONS OF OTHER ACFT; YET WE HEARD NO TFC IN THE BOZEMAN AREA. AFTER RECEIVING IFR RELEASE FROM SLC CENTER; WE BROADCAST ON FSS ADVISORY THAT WE WERE ABOUT TO DEPART RWY 30. I THEN BEGAN TO TAXI INTO POS FOR DEP. ABOUT THE TIME I WAS CENTERING THE ACFT ON THE RWY; A VOICE CAME ON THE ADVISORY FREQ AND SAID 'ACR X; DO YOU SEE THE SMA DEPARTING 12.' AT FIRST NEITHER MYSELF OR THE FO SAW ANYTHING. SUDDENLY; AGAINST THE BACKDROP OF THE MOUNTAIN; OVER A MI AWAY WE DID SEE AN SMA Y DEPARTING 12 AND HDG OUR WAY. IT TOOK OFF AND TURNED ON TO IT'S DEP PATH; PRESENTING NO FURTHER CONFLICT. WHY DIDN'T WE SEE IT? FACTORS. 1. NO RADIO RPTS FROM THE SMA Y OF IT'S INTENTION. 2. THE RWY AT BZN IS HIGH IN THE MIDDLE AND LOW ON EACH END. HENCE THE TAXIWAYS APCHING 12 AND THE DEP END ITSELF ARE NOT VISIBLE FROM THE DEP END OF 20.