Irish children's laureate Eoin Colfer and illustrator and writer Oliver Jeffers are both famous in their own rights.
Mr Jeffers, who grew up in Belfast, won the 2015 Book of the Year for Once Upon an Alphabet.
It is just one in a long list of awards he has received for his picture books.
Mr Colfer wrote the Artemis Fowl series of books that quickly became bestsellers.
Aimed at the young adult audience, the fantasy books focus on a young criminal mastermind who kidnaps a fairy in order to expand his empire.
He also penned the sixth instalment to Douglas Adams' The Hitchhiker's Guide to the Galaxy.
The former primary school teacher is from Wexford in the Republic of Ireland.
Both Mr Jeffers and Mr Colfer have recently worked with others in different art forms, but not with each other before.
Oliver Jeffers co-directed U2's video for their song 'Ordinary Love', while Kenneth Branagh has recently signed up to direct the film version of Artemis Fowl.
They decided to collaborate on 'Imaginary Fred' due to a chance meeting in New Zealand.
"We were there for the Auckland book festival and we met up at a story slam competition," Mr Colfer said.
"We were giggling like schoolboys at each other's stories, and at the end of the night we said let's do something together."
'Imaginary Fred' tells the story of Fred, who becomes the imaginary friend of Sam, a boy in need of company.
The two embark on a series of adventures together, but when Sam meets Sammi, a girl with an imaginary friend of her own, Fred has to move on from Sam.
The story, unusually, is told from Imaginary Fred's point of view.
"I like to do that with my books," said Mr Colfer.
"To take what is often a secondary character and make them the main character because they're a lot more interesting to me."
But with Mr Colfer living in Dublin and Mr Jeffers in New York, how did the collaboration work?
Mr Jeffers said modern technology made it all possible.
"Years ago we would have had to send carrier pigeons to each other," he said
"We used e-mails, phone calls, and, for showing visuals, I would hold my drawings up to the camera on the laptop via skype and we'd have a chat that way.
"Eoin could see everything I was doing in real time and we could talk about it together."
Despite their success working on their own, Mr Jeffers said they had no trepidation about working together.
"Eoin was an absolute pleasure to work with, and we both were very respectful of each others work," he said.
"We were just interested in making the best possible book, so there were no tiffs or fights or arguments.
"We were having fun, really, and I think that shows in the end product."
Now they've done it once both hope to collaborate again.
Their appearance at the MAC Theatre in Belfast sold out, and the venue was filled with young fans and their parents.
Mr Colfer said he thinks the interest in 'Imaginary Fred' shows that, despite all the other past-times available to children, interest in reading remains healthy.
"Every 10 years or so, somebody says that books are coming to an end - cinema was going to kill books, then it was TV, then e-books, but it's never happened," he said.
"Because nothing beats sitting down with your mum and dad and listening to your favourite story for the hundredth time."
Two of Ireland's most popular children's writers have joined forces to create an imaginary friend.