To specifically answer your question: You can not set your own restrictions for the FTP protocol. Each user gets their own home directory, and in this directory will be a logs directory, and if they are managing the files for a site, the HTTP server will store the logs there.
If you do not want a user to manage files for all of a site as well as run CGI scripts, then as Scott suggests you can “Remap Sub-dir” to have the server alias a url-path to a directory in the other user’s home directory.
Otherwise you can develop CGI or shell scripts that can ‘sync’ the files from the other user to the domain user given you turn off Extra Security and have appropriate UNIX permssions.
If the users are connecting via SCP or SFTP, you can turn on the Extra Security for the users so they cannot access each others files. But then you would need something like Rsync in order to transfer files via SSH.
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