The idea is to create two different object type groups. One for customer management and one for project management.
Each of these groups has a single folder type aka „context folder" - in your use case "customer" and "projects". If you want to create relations between customer folders and project folders, you can use reference fields. The main advantage of this concept is that there is no single truth - no hard-coded hierarchy. One customer can refer to many projects and one project can refer to many customers.
The references are resolved in real time and can be seen under the aspect "References" in the object detail view. In future versions, you can also use references in the structure service.