Stacking is perfectly fine as long as the units are assembled and used per assembly guides just make sure to attach units together using one of the following methods, based on your needs, to secure your modular creation in place.
This adhesive or hardware is not included, but is readily available at your local hardware store. We also really like the 3M mounting squares to secure your cubes together since they are removable. They're available on Amazon here. For a permanent option we recommend the gorilla tape available here.
Take a look at our Design Gallery to see how customers have organized and stacked. Please use precaution to secure units and anchor them to the wall to prevent any accidental tipping.