Symantec File Share Encryption offers real-time encryption\decryption of files stored on a secured File Share. File Share encryption happens on a file level where headers are added to each file in order to allow proper authentication of encrypted files with Symantec File Share Encryption.
The Shared Workbook feature incorporates Microsoft Office functionality so that multiple users can open a file, and make simultaneous edits to a file. Although this feature is very convenient, it has limitations as mentioned in Microsoft's documentation and does not operate as a typical Microsoft Office file, and using this feature on encrypted files may put the data at risk.
This feature is not tested, and not officially supported by Symantec File Share Encryption. Due to the nature of how file authentication is done with File Share Encryption, accessing and editing encrypted files at the same time with this Shared Workbook could cause unexpected behavior and using this functionality is done at the user's own risk. Furthermore, there have been issues reported that the Shared Workbook feature does not work as expected as outlined in the Microsoft documentation. Symantec has no plans to support this functionality at this time.
Please see Microsoft's article for more information on this feature.