From the NHC Glossary on the Fujiwhara Effect: "The tendency of two nearby tropical cyclones to rotate cyclonically about each other."
Its a rare event in the Atlantic. If two storms get too close, the stronger storm will either shear apart the weaker storm or simply absorb it into a feeder band (Cindy/Emily - 1999) depending on the location of each storm to the other.
If Julia should get too close, the outflow from Igor will weaken it with strong northerly shear. ED