Hi Inaki,
The additional files option is not available for File adapter with FTP transport protocol, only it is available for NFS transport protocol.
If you set additionalFile.optional parameter with "NO" then adapter will not pick up the first file only if the second file is not avaialable, adapter will pick up the files only if both are present, if not present the channel will raise an error with mandatory file is missing.
Regards,
Praveen.