I had only bought one set, but had informed myself beforehand and read about no restrictions that the system is not expandable. Only that components of older models (dLan) are not compatible.
I found the configuration a bit tedious (ok, I only read the quick guide afterwards ;-) ), but what helped me was the Devolo website itself (https://www.devolo.ch) and the support section with lots of questions and good answers. I had a quick look and found this answer to the question "Do Magic add-on adapters install automatically? "No. You have to add the new device with one of the previously installed devices by pressing a button. To do this, first select an adapter from the existing network, then please press the encryption button of the new adapter."
I can imagine that devices from another set are to be considered like an add-on adapter.
The Devolo Cockpit (also available for download on the Devolo website) may also help with the configuration.
Good luck!