This question comes up from time to time. The single bay My Cloud devices do not have an official way to backup its contents to a remote location. It will backup to an attached USB hard drive or local network location using Safepoint. See the following threads for more discussion on remote backup.
https://community.wd.com/t/setting-up-offsite-mirror-drive-for-mycloud/151187
https://community.wd.com/t/how-to-sync-myclouds-at-different-locations/150933
https://community.wd.com/t/off-site-backup/149927/
https://community.wd.com/t/re-enable-remote-drive-mapping-through-mycloud-com/134944
To have the remote NAS (Synology) backup to the My Cloud would involve having the Synology capable of remote backup, then configure the My Cloud to accept that remote backup (probably through configuring the My Cloud via SSH). One possible way, if the Synology supports FTP or SFTP is to have the Synolgy backup its contents to the My Cloud via FTP/SFTP. One can enable FTP on the My Cloud via the Dashboard. One would also have to configure their router the My Cloud is connected to for Port Forwarding of the FTP/SFTP port to the My Cloud.
