When mapping an s3 bucket to a drive, under advanced options, there is a setting for "Microsoft Office Files lock control" that is supposed to prevent multiple users from being able to write to a single file at the same time. Per the description, if a user is editing a document, other users will only be able to open a read-only copy of that document. Unfortunately, enabling this option does not prevent multiple users from accessing the document at the same time, and whichever user's edits are saved last will overwrite the other user's edits version. (I am not sure that it matters, but my s3 bucket has versioning enabled).