How can I backup/synchronize between two FTP servers?

You need to use your local drive as an intermediary storage device.

Create two profiles: one to backup/sync with FTP server 1, and one to backup/sync with FTP server 2. Next, create a group profile and put both of these profiles into it (in the appropriate order). Now, run/schedule the group profile. Note that this will only work if both servers can set the date & time of files on the FTP server. You may need to experiment to get the settings correct.

Always test first using files you can afford to lose!