I was looking at this same question but in reverse, I upgraded from the 321 to a DS211. (Huge improvement in speed, easier to administer and far more functionality built in, btw, but at $$ cost). My question was about using the 321 as a backup target with an automated process that runs on one of the two devices and calls the other without having to run everything through a PC. Synology has an app that should do that but it requires that the target (the 321) have an rsync module running, something that I guess requires some funplugging and that's as far as I've gotten. I haven't figured out whether the 321 scheduled backup will do the job with a "pull" from the DS211, but it looks like it can back up from a network resource like you've indicated and that's the direction I want to go.
Have you looked at the Synology forum? There's a board dedicated to backups, they might have some advice for you there.
As has been pointed out, the problem with all of the PC software is that it requires a third machine to be running, and also slows the whole thing down quite a bit. And the 321 is no speed demon...