Assumed the shared note also contains the unique note id then the receiver can import it and subsequently edit it. Once he finished his changes he could share his modified version back. If the sender didn't change his copy since he shared it, the new version just replaces his one. If both parties concurrently changed the shared note, a conflict could be indicated like it is done with conflicts during synchronization.
Something like this?
BTW, with sharing I mean any method that would allow a note to be transferred. E-Mail, Messenger, direct WiFi connection, Bluetooth, etc. Just anything the a smart phone's or desktop's OS provides.
If it was as easy to share Joplin notes as it is with other documents/files (e.g. PDFs, Images, etc) then one would have the asynchronous workflow you outlined above for free. The intermediate server you "published" to would then be an email sever, a messenger server, etc.