When two hurricanes collide, the phenomenon is called the Fujiwhara effect. If two cyclones pass within 900 miles of each other, they can start to orbit. If the two storms get to within 190 miles of each other, they'll collide or merge. This can turn two smaller storms into one giant one. In rare instances, close proximity can throw a storm off course, as was the case with hurricanes Hilary and Irwin in July 2017.