How can I write a creative conversation between two ghosts that occupy the house in "The Listeners"? The dialogue should explain the story behind...

This sounds like a really fun writing prompt. There is a lot of freedom for you to write about various things. The poem itself reads a bit creepy. A lone traveler arrives at a house at night. He knocks on the door and is greeted by silence. The poem mentions "phantom listeners" within the house, which is definitely eerie.

But only a host of phantom listeners   

   That dwelt in the lone house then 

Stood listening...

This sounds like a really fun writing prompt. There is a lot of freedom for you to write about various things. The poem itself reads a bit creepy. A lone traveler arrives at a house at night. He knocks on the door and is greeted by silence. The poem mentions "phantom listeners" within the house, which is definitely eerie.

But only a host of phantom listeners   

   That dwelt in the lone house then 

Stood listening in the quiet of the moonlight  

The poem doesn't explain why the traveler is at the house. All readers know is that the man made a promise to show up.

‘Tell them I came, and no one answered,   

   That I kept my word,’ he said. 

For your dialogue, I first recommend that you choose a genre. Is your dialogue going to mirror the creepy and eerie mood of the poem, or is your dialogue going to go a completely different direction? The prompt says to be creative. I believe that gives you the freedom to change the mood of the dialogue completely. I would write a comedy. Have your ghosts crack jokes about the traveler. They know why the traveler is there, but perhaps they are hiding from him in order to play a joke on him. The traveler doesn't state that he has been prompt in keeping his promise. Perhaps it is years later that he finally shows up. The listeners have grown tired of waiting for him, and they decide that the man deserves some payback. They will make him wait.

To make your ghosts funny and realistic-feeling, I would base them in reality. Pick your two funniest friends. Imagine that they are the ghosts, and write the conversation while thinking about how your friends would actually speak and act in that situation. You could even use the names of your friends for the names of the ghosts; although, I prefer Casper and Sir Simon for the names of the ghosts.