2-way rsync?

Think of rsync like a process that erases everything in the destination and copies what's specified on the source to a destination folder, depending when it's executed you can't be sure of the results if you use several jobs for that. It's not that way Rsync works but the result is the same. However what you want to achieve is not a 2way sync but two different one way backups. One to save every day, and one to restore if you get a crash. This is possible with Rsync quite easily.
Rsync can be used to a folder and doesn't require an rsync server, which means that if your older NAS hasn't an Rsync server nor client, you can always mount an smb share and rsync to it from the 312 (just reverse source and destination to restore), I would advise some scripting to check if everything worked fine though. This method requires SSH access which may or may not be supported by Netgear (they say they don't support it, but I think that disclaimer is only when you do really bad stuff).
That said, the 312 can backup to an SMB share using timestamps of files and folders, that would require a modified file to be uploaded in full each time but if you do it by night this could be good enough and it's supported by GUI.