THE ACFT CRASHED DURING AN EMERG NIGHT LANDING AFTER THE LOSS OF PWR ON BOTH ENGINES. INVESTIGATION REVEALED THE RT ENG WAS SECURED BECAUSE OF DECREASING OIL PRESSURE & A VISIBLE OIL LEAK. THERE WERE SEVEN AIRPORTS WITHIN REACH OF THE ACFT WHEN THIS ENG WAS SECURED. HOWEVER, THE PLT INFORMED THE ATC SPECIALIST AT NORTHWAY THAT HE INTENDED TO CONTINUE ON TOWARD HIS FAIRBANKS DESTINATION & WOULD LAND AT BIG DELTA IF HE HAD FURTHER PROBLEMS. THERE WERE NO SUITABLE LANDING AREAS BETWEEN TANACROSS & BIG DELTA, A DISTANCE OF 74 MILES. LATER WHEN THE PLT SECURED THE LT ENG, HE ATTEMPTED TO REACH THE ALASKA HWY BUT CRASHED IN A WOODED AREA. EXAMINATION OF THE ENGS REVEALED THAT BOTH ENG BREATHER TUBES WERE FROZEN FULL OF ICE FOR 5 INCHES ABOVE THE NORMAL DISCHARGE OPENING. THE ENG NOSE CASE SEAL WAS BLOWN OUT ON THE RT ENG. THE OIL COOLER ON THE LT ENG WAS RUPTURED. THERE WERE NO PRESSURE RELIEF HOLES IN EITHER OF THE ENG BREATHER TUBES/PIPES. AN ALTERNATE BREATER TUBE WAS NOT INSTALLED.