Often said of people who meet for a brief but intense moment and then part, never to see each other again. These people are like two ships that greet each other with flashing lights and then sail off into the night. From a poem by Henry Wadsworth Longfellow.
ships that pass in the night
Individuals who are rarely in the same place at the same time. For example, Jan works the early shift and Paula the late shift
they're two ships that pass in the night. This metaphoric expression comes from Henry Wadsworth Longfellow's poem "The Theologian's Tale" (published in Tales of a Wayside Inn, 1873).