Hi Maljaars, Nico,
Thank you for posting in Q&A forum.
Co-authoring Office 365 documents works best with OneDrive, SharePoint Online, and other cloud-based solutions because these platforms are designed to support real-time collaboration. Unfortunately, co-authoring features are not fully supported for documents stored on traditional network drives accessed via UNC paths or mapped drives.
When documents are stored on a network drive, you might be able to edit them sequentially, but not simultaneously. This is because network drives lack the collaborative features and conflict resolution mechanisms built into cloud services.
Here is a post with similar requirements for your reference:
I hope the information above is helpful.
Best regards
Zunhui
============================================
If the Answer is helpful, please click "Accept Answer" and upvote it.