Customer owncloud

When working on a document via a desktop client, the file is not blocked, i.e. another user can open and edit it

I suppose you are using the text editor from ownCloud.

When a File is used by any app or syncing. it will be locked in the Redis server or in the database until it is released.

If you want to work simultaneously you have to install collabora or onlyoffice who wives you the change to collaborative working.