Argthtjtr

I have been looking through various books with a dungeon being the main character.
Regardless of the book, however, the character had to gain a physical body in order to forward the plot, which I didn't want to write.

As a solution, I got together with a friend and we decided that I would write the dungeon part while my friend wrote the part of a character who advanced the plot out of the dungeon.

How can we organize the book while both of us write different characters?

I am currently publishing a comic series with a co-writer, and have worked with other writers as well.

We always start with a general outline, so that both of us are in agreement regarding what direction the plot is going to take and how the characters are going to advance it.

Typically we each follow our own self-created characters and their story arcs, although our characters also interact so we end up writing each others'. We also make suggestions for ideas involving each others' characters as well.

Then, each of us write our designated chapters, after which we submit it to the other author to approve. Sometimes this involves some back and forth until we reach agreement.

It's easier in my case as I'm the primary writer so have ultimate say, however if my partner felt passionate about a particular issue I would be inclined to follow her lead. Co-writing doesn't work without compromises!

The simple approach.

All you need to agree is what is the status quo at the end of the first part, which, incidentally, will be the status quo at the beginning of the second part.

It is like a journey in which you take a coincidence somewhere. You need to be at a certain place at a certain time in order to continue. Similarly, define where characters are, what they are doing, what they know. The first part of the book should describe how they got there. The second part of the book should deal with where they go from there. How you organize the text within each section is up to you.

A less simple approach.

Each of you can write their section as they please, you could make whatever assumptions you like about the plot prior to your section of the story. Your friend can similarly decide to take whatever course they wish for their part of the story.

Once you are done, you can sit together and write a few more chapters to bridge between the end point of one section and the beginning of the next. This is actually quite fun and it may require some good creative thinking.

An even less simple approach.

Both of you write a very skinny draft, just outlining the main story. Even a summary of different scenes should suffice. You should mark details that you are aware of and that occur during each scene in this draft. These details may need to be known across the two sections of the book. After this, you write a second, longer draft of the two sections. Again, make sure you exchange every detail of lore, or character characterization that you have added to make your section consistent: the other writer may need to reference them in their section.

A third draft is probably going to be needed, but beyond that, it is just revisions.