There are three options:
You simply specify to connection and folder information for both sites and that's it. It will keep two sites in sync automatically. You even don't need to install anything on your own computer. It's an online service.
For more information, please visit https://www.syncbear.com
If one of servers is running Windows system and you have permission to do that, FTP Synchronizer will work for you without problem. Just like normal local and remote sync.
For example: There are two folders: folder_a and folder_b in site_a and site_b respectively and you want to keep two folders in sync. Here are what you should do:
After running two profiles, site_a and site_b will be synchronized.