COMMUTER ACR AT INTXN OF TXWY L AND RWY 16 TOOK TKOF CLRNC FOR COMPANY ACFT AT THE DEP END OF THE RWY. BOTH ACFT TOOK OFF AND ERRANT ACFT HAD GND NMAC WITH A WDB AT TXWY N AND RWY 16L.

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UPON CLRNC TO TAXI TO RWY 16L AT SEA; WE REQUESTED INTXN 'L' WHICH IS APPROX HALFWAY DOWN THE RWY. WE WERE FLT ACR X ABAG. AFTER HOLDING SHORT OF RWY 16L AT 'L' WE BOTH HEARD 'ACR X BAG CLRED FOR TKOF; TURN R HDG 130 DEGS AFTER 1000 FT; MAINTAIN 3000 FT.' I'M NOT SURE; BUT I BELIEVE I ALSO HEARD IN THAT CLRNC SOMETHING PERTAINING TO INTXN 'L.' THE FO READ BACK THE CLRNC AS WE HEARD IT AND WE DEPARTED. AT 700 FT THE TWR ASKED IF WE; ACR X BAG; WERE AIRBORNE; AND WE RESPONDED YES. THE TWR THEN SAID FLY RWY HDG; MAINTAIN 3000 FT. AT THIS TIME TWR INFORMED US WE HAD TAKEN OFF WITHOUT A CLRNC. IT WASN'T UNTIL AFTER WE HAD REACHED 3000 FT THAT WE HEARD ANYTHING OF ACR Y CCG OVER THE RADIO. SHORTLY AFTER REACHING 3000 FT WE RECEIVED ANOTHER HDG AND ALT FROM ATC AND CONTINUED WITHOUT FURTHER INCIDENT. I KNEW NOTHING ABOUT THE PROX OF ACR Y CCG WITH US UNTIL THE NEXT DAY WHEN I CALLED SEA TRACON. TWR'S CONTENTION IS THAT WE HEARD ACR Y CCG CLRNC AND TOOK OFF AFTER READING BACK THE CLRNC WITH 'ACR X BAG' AUDIBLE AND THE REST GARBLED. ACR Y CCG WAS AT THE BEGINNING OF RWY 16L AND WE WERE AT THE 'L' INTXN; JUST ABOUT HALFWAY DOWN THE RWY. FROM WHAT I KNOW FROM OUR POINT OF VIEW AS THE CREW; AND FROM WHAT I HAVE BEEN TOLD BY SEA TRACON; THIS INCIDENT WAS CLRLY CAUSED BY MISCOM OVER THE RADIO -- PERHAPS BY BOTH US; THE CREW; AND SEA TWR. A POSSIBLE DETERRENT TO THIS SIT COULD BE TO RESTRICT INTXN TKOFS BELOW A CERTAIN VISIBILITY HIGHER THAN 1/2 MI WITH RVR AT 1200. SUPPLEMENTAL INFO FROM ACN 261597: NEAR THE END OF LNDG ROLL; RWY 16R; WE ADVISED THE TWR WE WISHED TO USE TXWY 'N' TO EXIT RWY 16R. WE WERE CLRED TO DO SO AND TO CROSS RWY 16L. WHILE ON 'N' WITH THE ACFT NOSE APPROX 150 FT FROM THE W EDGE OF RWY 16L; AN ACFT WAS OBSERVED IN ITS TKOF ROLL AS IT PASSED IN FRONT OF OUR ACFT. SUBSEQUENT DISCUSSION WITH THE TWR SUPVR (TELEPHONE) REVEALED THAT A TKOF CLRNC FOR AN ACFT DEPARTING RWY 16L WAS ACKNOWLEDGED BY 2 SEPARATE TRIPS AND THAT THEIR SIMULTANEOUS RESPONSE BLANKED EACH OTHER OUT. ONE BEGAN ITS ROLL FROM THE END OF RWY 16L; THE OTHER FROM AN INTXN OF RWY 16L. THE TWR'S EFFORTS TO CORRECT THE SIT WERE NOT SUCCESSFUL AND BOTH ACFT DID TKOF ON RWY 16L AT NEARLY THE SAME TIME. I AM OF THE OPINION THAT THE TWR OPERATOR ISSUED A PROPER CLRNC TO ONE OF THE 2 AND THAT GND SEPARATION BTWN HE AND I WAS ADEQUATE. I AM ALSO OF THE OPINION THAT THE POLICY OF ALLOWING INTERSECTIONAL AS WELL AS FULL LENGTH TKOFS FROM THE SAME RWY DURING PERIODS OF REDUCED VISIBILITY (CAT II IN THIS CASE) SHOULD BE REVIEWED. THIS PRACTICE IS QUESTIONABLE; IN THAT COMBINED WITH RADIO CONFUSION; EXPOSES EVERYONE INVOLVED TO A DANGEROUS SIT. IN VFR CONDITIONS CHANCES ARE IMPROVED BECAUSE OF THE 'EYEBALL FACTOR.'