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Hi @Camille,
Since the workbook is stored in a SharePoint Online library but both users are opening it through a synchronized folder in the Excel desktop app, I would first check whether the issue is related to how the workbook is being opened rather than replacing the file each time.
Please try the following with the same workbook:
- Close the workbook on both computers.
- Open the SharePoint library in a browser.
- Open the workbook and choose Open in Desktop App.
- Confirm that AutoSave is turned on, then have both users make a small test change and verify whether the changes are saved to the same workbook.
Microsoft notes that AutoSave may not be available as expected when a cloud file is opened from File Explorer/Finder or from a recent-file entry that points to a local path. Reopening the file from its OneDrive or SharePoint location is one of the recommended checks: Turn on autosave in Microsoft 365 apps | Microsoft Support
Also, on both computers, please confirm that:
- OneDrive synchronization is running and is not paused.
- The workbook is in a supported format such as .xlsx.
- In Excel, go to File > Options > Save and confirm AutoSave OneDrive and SharePoint Online files by default is enabled.
More information about conditions that can prevent AutoSave from being available can be found here: What is AutoSave? | Microsoft Support
For now, I would avoid creating a new copy and replacing the workbook, as that can make it harder to determine why AutoSave is being disabled.
If the issue still occurs when the workbook is opened directly from SharePoint using Open in Desktop App, please reply with:
- The Excel version/build from File > Account > About Excel on both computers.
- Whether AutoSave turns off immediately when the file is opened, or only after the workbook has been open for some time.
- Whether the same behavior occurs with a newly created .xlsx workbook stored in the same SharePoint library.
Those details should help narrow down whether this is specific to the original workbook or to the Excel/OneDrive environment.