Only sync file once another file got updated on FTP

[extremecarver]

Hi, I have a rather difficult to solve problem I guess - not sure if it is possible.

I have some files which should only be synced - once a corresponding file got updated on a remote FTP server.
Basically it's a pair of files. File A and File B. While File A is quite small, File B is very big and therefore not offered on the same server as File A.

File A gets synced via sftp from a windows server to a linux server. Once per night another server will rsync file A from the linux server and offer it to download via FTP.
Only once that sync is done - File B should be synced. The original server of both files runs Windows. The sync server runs linux if that is of any concern. I don't know how to detect when the rsync daemon accessing the file A finishes (it does grab loads of files, and sometimes it runs for 10-12 hours until it has finished rsyncing all files)

Any idea how to do this properly? Right now I just set File B to update around 5AM - hoping that File A is by that time downloadable from the FTP server. Sometimes 5AM is too early, sometimes too late - and sometimes the rsync job is not happening for a full day and the file pair therefore is out of date for over a full day.

Any idea how I can setup freefilesync to only sync file B once file A is accessible via the FTP server (doesn't need to be instantly, but a check every 30minutes or so would be great)?