Okay, this would be a lot prettier if I had my husband cut lumber for me to the right size. I have stacked the cabinets like this before and had the right-sized lumber at that time. Today I didn't want to really stack my cabinets but wanted to show a friend basically how it's done. I grabbed scrap lumber from the basement and pieced it together to show the general idea.
So, the Montessori cabinets (botany, geometry, pin maps, etc.,) are inexplicably made unstackable. There are a couple different variations on shape for the top "walls." The cabinet above (originally a geometry cabinet) has curved side walls. Pin map cabinets have two-level side walls just in the back. But, you can stack them if you want to.
You want to set it up so that you aren't using the upper OR lower side walls for support. You are going to create a set of walls inside those walls to use instead. I grabbed 2x4 lumber from the basement. Placed on its widest face it is just a little bit too short and the width wastes what could be a useful storage cubby later when the cabinets are stacked. Placing the narrow face downward makes much better use of the resulting cubby but the lumber is then too tall. It works, but is unattractive. When I did this for real I placed the lumber on it's narrow face (the 2" face) and cut the 4" measurement shorter. I think to about 2.5 or 3 inches. I also nailed the u-shape together for stability.
This is how it will look from the side.
You could keep doing this successively. For example, if you bought all of the pin map cabinets by continent and wanted to stack them all. If it gets very tall you will want to anchor them in some way for safety. I will probably do this again next year when I buy the equivalence material and Pythagoras plates. I intend to buy a second geometric cabinet just for the cabinet so I can store those.