NAS to NAS backup

I can see it's been awhile since you posted your message. Are you doing a disk/partition (Computer) or files/folder type backup.. It would seem to me that if it's going to work it's going to be the files/folders method. Once you do an initial backup then set up a scheduled differential or incremental backup.

If that does not work you may want to try and map a network drive to your NAS and try it that way.

I don't know about your setup but I use two NAS backups for our servers where I work. I do daily backups to one NAS and then on Mondays I swap it out with the other NAS. One stays online and the other goes in a fireproof safe.. It could go off site as well.. This way I have up to two weeks of backups minimizing the chances of a corrupt backup

There are other ways to do what you want.. One would be to synchronize both of the NAS boxes. There are a number of free or almost free softwares to do this.