Yes, any number of smartphones can be connected to the Anio watch.
Once the Anio is registered, you can log into the "Anio Watch" app on a second smartphone using the same login credentials (email address and password).
The watch is only registered once. All other family members install the app and log in with the same data (email/password) used during registration. In the right app card menu under USER DATA, each of you then assigns a chat name (Dad, Mom, etc.) for the chat history.
For security reasons, the Anio can only be registered once. If you plan to sell your Anio, delete the watch from the app via the device menu.
It can then be newly registered with a different email address.