So let's say you have user A and user B. If you want to copy a file from user A's "outbox" folder to user B's "inbox" folder. You can do it one of two ways.
Either user B can make user A a collaborator with Editor access to the "inbox" folder. Then from user A's account they can do the copy API (POST /files//copy documentation here) and "push" the file into B's inbox.
Or
Via a "pull" model. User A can give user B access to their "outbox" folder, and user B can run the copy API, and copy the content to their "inbox" folder.