One has a number of options if both NAS devices are on the same local network. From using software on a computer to compare each folder location and synchronize between the two NAS devices to seeing if GoodSync may synchronize between supported NAS devices to using Rsync (and CRON) scripting to copy files from one location to another.
It will likely come down to the specific NAS models one is using and what options and features each support. Some of the more full featured NAS devices, like those from Synology or Qnap may have apps/modules that will sync files with another NAS device. The OS5 My Cloud does have an option for backing up to another supported OS5 My Cloud.
My Cloud OS 5: Remote Backups
https://support-en.wd.com/app/answers/detail/a_id/30036
GoodSync for WD NAS Users
https://www.goodsync.com/for-wd-users
GoodSync Free and Premium for WD My Cloud
https://support-en.wd.com/app/answers/detail/a_id/29077
Rsync to Synology Drive
https://community.wd.com/t/rsync-to-synology-drive/255560
Free File Sync
https://freefilesync.org/
