What has to be true for it to work
Co-authoring depends less on the editor than on where the file sits. In Microsoft 365 the document must live in OneDrive or SharePoint, use a current format such as .docx, .xlsx or .pptx, and be opened by people who each have edit permission; Microsoft lists Word and PowerPoint on every device and the current Excel for Microsoft 365 as the supporting apps. Coloured flags mark where each colleague is working. Google Docs, Sheets and Slides are built on the same premise from the start: once a file is shared with editors, everyone works in it together, with comments, suggestions and a chat inside the file.
- A file emailed as an attachment is a copy, and no longer part of the shared document.
- An older file format or a document checked out in SharePoint stops simultaneous editing.
- **Encryption from a sensitivity label** blocks desktop co-authoring unless an administrator switches on the tenant setting that allows it, and even then some encryption options remain excluded.
- Working offline keeps a person's edits invisible to the others until they reconnect, which is where conflicts appear.
The safety nets around it
The safety net that makes simultaneous editing tolerable is version history: if a colleague deletes a section by mistake, the earlier state is a restore away rather than lost. For documents that need a formal review cycle, track changes or suggesting mode turns edits into proposals, and many teams switch it on once a draft moves from writing to approval. The Microsoft notes also carry a practical limit: co-authoring on files kept in Google Drive is not supported from the Office apps. The collaboration and meetings topic treats co-authoring as the default way shared work happens, and the file-sharing decisions around it as the skill.