By default, without the physics, it is possible to add multiple ‘bounding box’ shapes. Not so with Physics 2. Along with the inability to make ‘concave’ shapes, these can be a problem with my game. For example, how can I have cargo sit flat on the back of a truck?
So is your question “how do I get multiple physics 2 body shapes”?
To answer your truck and cargo example, break the truck up into it’s part - deck, cab and wheels. Make the physics 2 shape a box for the deck, and also for the cargo. Increase the friction for both.
Or have I missed what you’re asking?
Nope, that is it! 
Either that, or more complex shapes.
I figured that was the way to do it, too bad, oh well.