Yeah, I kinda missed that after the first cut, repeat the plane cut and slide the tool higher or lower each time, using the graduated scale. When you grab the vertical arrow with the mouse, a number appears. It appeared in this model at the 40 mm point, and I dragged it downward.
If you forget to use the Keep both option, hit Control-Z and get the model back and start again at that step.
You can import a sphere into Meshmixer and experiment with a manifold shape, to see how multiple slices will be created when the Separate shells option is used.
Once you have the shells, go into preferences and file, and make sure the option is set to save only the selection. You can then pick each piece, export it as stl and have your individual components.
I’ve used this multi-step method to slice large models for a friend with a smaller bed printer with great success.