explain the bonding between the captain and leggatt in the secret sharer?

The Captain refers to Leggatt as his "double" throughout the story, and the fact that the Captain now finds someone with similar qualities to his own and has someone to talk to make it seem like a romance. The way the two communicate can also be seen as romantic. They tend to whisper to each other in a fairly close space. The romantic actions continue at the end of the story when we see how the Captain truly cares for Leggatt. The Captain is willing to get dangerously close to shore in order for Leggatt to reach the island safely. This caring act further displays the powerful bond between the Captain and Leggatt.