Public Folders should be available per organization and NOT per domain. In a mutli domain organization you won't be able to work together using Public Folders. Thanks. 
Public Folders availability per organization
Sounds great, but this is nothing that can be implemented without breaking our drop-in-compatibility with MSEX and M365, since in EX-realms there is only a single PF-root per installation(!) possible. In M365, this theoretically is possible which is why we are going to investigate this in the future.
Sorry to break the news to you, this will not happen any time soon.