That is very helpful, thank you!
Just a clarification: as it is now, with the 'secondary' Photo Library resident in the MacBook Air (with iCloud Photo Library activated) i can access the photo in the library, and also the photos in the 'main' Photo Library in the external disk.
Question: when i designate as default library for the MacBook Air the Photo Library in the external disk, why do i need to have direct access to the files (i.e.: having the external disk connected to the Macbook Air) to see the photos, and i am not instead able to see them through iCloud Photo Library, as int happens now? Is this because the new default library for the Air is not reachable?
To overcome this, can i designate for for the Air a new library, resident in the Air, connected to iCloud Photo Library (a semi-empty library, just to activate the iCloud Photo Library)?
Thank you again.
M.