SYDNEY, Feb 15 (Reuters) - Two tropical cyclones are bearing down on three South Pacific island nations creating a "critically dangerous situation", weather officials said on Tuesday. Cyclones Olaf and Nancy, which are expected to hit Samoa, American Samoa and the Cook islands, could cross paths creating one giant, destructive storm centre where the two cyclones spin around each other until one is flung off. "For the South Pacific it is unusual to have two cyclones this close together," said Kevin Vang at the Australian-Pacific Centre for Emergency and Disaster Information. "This has the making of an absolute mess,"...