The Section Plane is a powerful SketchUp feature. You can cut your model anywhere in realtime. You can create as many section planes as you like, but SketchUp will only allow one single active section plane at a time (in the same context, as we will see). To use simultaneously active planes, the trick is to group your model with a section plane and create another one outside the group.
Create the first section plane, and place it at the desired position.
To avoid letting any part of the model out, double-click the first section plane to de-activate it. Select the model parts and the section plane and create a group. Now create a new section plane. Edit the group to activate the first section plane.
Now you have a model with two simultaneously active section planes.
This can be used as a presentation tool, to explain internal features of your model.