The actor said he calls the pair the "Bickerson Brothers" because of their constant fights.
Harrelson has been writing a screenplay with Owen, but admitted they were struggling to get down to work.
The Daily Record quotes him as saying: "We have written about three sentences. It's just so much fun hanging out with Owen, it's hard to get down to business. I like to bring in the other Bickerson Brother too. That's what I call the Wilson brothers. I consider myself one of them too."
He added: "We bicker when we play bocce or croquet. You can't believe how serious a wonderful lawn game can become when you play it with the Wilson brothers."
The 46-year-old recently took close friend Owen on a trip to Peru to recover from his suicide attempt last August.
He can currently be seen in Coen brothers thriller No Country For Old Men.