I do not believe there is a GUI solution, at least not one with direct traffic between the NAS devices.
The one I can think of, will require you do some linux commands.
http://community.wd.com/t5/My-Book-Live/GUIDE-Enable-FXP-transfers-between-2-MyBookLives/td-p/535436
FTP directly is another option, but then, it would be just like rsync. And needs to be done via CLI.
The simplest solution would be to map a drive for each NAS and copy/paste. However, like I said, data will flow as follows:
MBL → PC → Sysnology.
rsync would be my recommendation. Actually, keep the MBL as a backup for the Synology for the VERY important data. You can use rsync for it.
http://www.thegeekstuff.com/2010/09/rsync-command-examples/
Also, just want to share this link with you, just want to make sure you understand the difference between redundancy and backup.
http://www.cnet.com/how-to/digital-storage-basics-part-3-backup-vs-redundancy/
