THE ACFT WAS 1 OF 5 THAT WERE ON A RETURN TRIP FROM LOS ANGELES, CA, TO ABBOTSFORD, BRITISH COLUMBIA, CANADA. THE FLT CREWS HAD OBTAINED WX INFO DURING STOPS AT CRESENT CITY, CA, & NEWPORT, OR. AFTER LNDG AT EVERETT, WA, ONE OF THE FLT CREWS FILED A FLT PLAN FOR ALL 5 ACFT. AT THAT TIME, HE WAS ASKED IF HE WANTED A WX BRIEFING, BUT HE INDCD THEY HAD ALREADY RCVD A BRIEF FOR THE ROUTE. RPRTDLY, THE PLTS ALSO OBTAINED WX INFO FROM A COMPUTER IN THE LOBBY OF AN FBO AT EVERETT. AFTER DEPG EVERETT ON A NGT FLT TO ABBOTSFORD, THE FLTS ENCTRD DETERIORATING WX WITH SNOW & CLOUDS. THE FLT CREWS OF 2 ACFT REVERSED COURSE & LNDD AT AN ALTERNATE ARPT. THE CREW OF A 3RD ACFT DIVERTED, WENT OVER WATER, THEN CONTD EN ROUTE. HOWEVER, THE 2 REMAINING ACFT (C-GIXI & C-GJNM) COLLIDED WITH MOUNTAINOUS TRRN ABT 3 MI APART. C-GIXI IMPACTED ON ROUGH ROCKY TRRN AT AN ELEV OF ABT 1200'. ABT 8 MI NW AT BELLINGHAM, WA (ELEV 158'), THE 1950 WX WAS IN PART: 2100' BKN, 10 MI VIS WITH RAIN, WIND FM 240 DEG AT 6 KTS. THE WRECKAGE WAS FND THE NEXT DAY CVRD WITH SNOW.