You can sync 2 folders of the same NAS using the "TASK SCHEDULER" of DSM => see attached picture
You create a task that executes the linux Shell command here below, and define when this task is executed.
This is a pseudo realtime sync, since it runs on a periodical basis (I have a sync every 10 min)
rsync -a "/volume1/sharedfolder1/.../folder1/" "/volume1/sharedfolder2/.../folder2"
This command Synchronizes the CONTENT of folder1 with folder2
Any change made in the content of folder1 is reproduced in folder2 and vice-versa
* The "..." can be replaced by any folder path going through your folder structure, you must always start with a shared folder at the root of your volume (here "sharedfolder1")
* Be aware of "/" at the end of "/volume1/sharedfolder1/.../folder1/", and not present at the end of "/volume1/sharedfolder2/.../folder2". This indicates that you sync THE CONTENT OF folder1.