For the past three years, the two of us have been working on a five-book epic fantasy series called Magisterium. The first book, The Iron Trial, comes out this month. Its publication has caused us to look back and reflect on the process of co-writing and how the two of us put aside our own solo projects to create something together.

Both of us come from a background of collaborative projects. For the past two years, Cassie has been working on The Bane Chronicles, a series of novellas set in her Shadowhunters universe, co-written with Maureen Johnson and Sarah Rees Brennan. Holly's first middle grade series, The Spiderwick Chronicles, was created and plotted out with Tony DiTerlizzi. And both of us have written several short stories together.

But none of those things quite prepares you for sitting down to the kind of co-writing that we've done on Magisterium. One of the most common questions we get when talking about The Iron Trial is about the actual process of collaboration. People want to know the specifics of how it's done, what parts each of us write, and how we settle disagreements.

Our co-writing process is admittedly unusual. In most cases when books are co-written, authors alternate chapters, or each will take a viewpoint character. Which makes sense, because usually, when authors are co-writing, they aren't in the same geographical space.

The two of us happen to live less than a mile from each other. Our process is very keyed to our presence. We sit together with one laptop, and the first writer, say, Cassie, will write out a scene. Then she'll pass the laptop to Holly, who will go over the scene, edit and smooth it, and then write the next scene. She'll pass the computer back to Cassie who will do her own changing and editing. Therefore every scene is in essence, written by us both.

When we tell people that we often get surprised or even uncomfortable reactions. According to Lorraine Mary York's Rethinking Women's Collaborative Writing: Power, Difference, Property, "Critics and readers feel a persistent need to 'de-collaborate' these works, to parse the collective text into the separate contributions of two or more authors."

The idea that you cannot tell who wrote what — that sometimes even we can't tell — I think challenges the idea of the ownership of a text. If one single person doesn't own the words, then who does?

To us, of course, it's obvious: we both do. But it's taken us a journey of being able to let go of the idea of complete ownership over a work. Writers are used to being the gods of their universe. They're used to being able to control everything that happens in a story. When you enter into a collaborative process like this, you have to give up on that idea. Your sharing of the story has to be total and you have to accept the characters as belonging not just to you. It's a definite transition.

But as Holly says, "What I love most about co-writing is that you can put your insecurities and ego aside and just work on the story itself." Once you have completed the process of letting go, there's something very freeing about sharing a story and characters. You aren't the only one shouldering the burden of the choices about what happens to them. You aren't the only one tasked with coming up with every detail. And most importantly in those days before publication, when readers come to share the story with you, you aren't the only one who cares.

What we started with was a love of these characters and the idea that we wanted to take reader's expectations of what happens in a book series with a magic school and twist them. Whenever we sit down to brainstorm what happens next to Call, Aaron and Tamara, we're always excited.

Whenever we write, we're eager to get to this part or that part. These books exist outside of our solo series and are allowing us to try out being slightly different writers – or possibly, a single writer who is, hopefully, the best of both of us.

Our goal has been to create that sense of one seamless voice – the voice of Magisterium novels – in which a reader can trust not to falter or vary. We wrote the books much like we wrote this article. Can you tell which of us wrote which part?

• Cassandra Clare and Holly Black's Magisterium: The Iron Trial is available from the Guardian bookshop.