In your case I recommend to use Entware-ng
to install rsnapshot
to do the job. Rsnapshot has as advantage above simply synchronizing that is takes incremental snapshots which behave like full backups, without using more diskspace. (When a file is in different snapshots, it will be on disk only once. Using hardlinks the different entries point to the same disk space. Only when the file changes on the source, a new copy is made.)
For this scenario you should not write to the snapshots directory. If you really want to use both volumes and sync them (where there will be a problem when a file is changed on both volumes between 2 syncs), you can use rsync. (Also available in Entware-ng).
To automate the job you can use cron. The cron in the firmware can be used using Tweaks