I wanted to add 2 more journals to my MTNKO. I saw a blog post somewhere, I cna't remember the blog that added a additional pockets to the notebook through the use of a rubber band. I decided to use that idea to add 2 more notebooks. I wanted to have my general idea notebook, a notebook for PioP and another sketchbook. To do this hack you'll need the following:
A medium length thin rubber band or a loop of the elastic you used to make you MTNKO
Slide the rubber band through the center of one notebook then the other,so they are attached spine to spine. Slide one notebook under the notebook already in your MTNKO.
Here's a helpful video of the process.