There is a lot of planning that goes into writing with a partner. Our first book involved a lot of matching up timelines, dates, and times to assure we didn't end up confused.
We took to making story boards. Our first was done with sticky notes and different colors. It was a ton of work, but helped us see how each side of the story was progressing.
We used it to see which day the characters were at, when interactions would happen, and what the weather was like during each event.
I loved making this, it was so helpful, but it wasn't very practical. It used a ton of sticky notes and they kept falling off... So the next one, we went with just writing on the poster board with different color markers.
We didn't lose anything there... but if we made a mistake it meant scribbling out the words or starting over. That too wasn't very productive.
Our newest method? A white board! Different colors for each character and dates and such. Easy to erase without anything falling off. We've only done one story this way, but it seems to be working well. We'll see what the future holds... but for sure, it will be more story boards.