There is an NVR option to add network storage under the Disk menu object, Storage Device, Add- either NAS or IP SAN. But the Storage Mode is where you setup groups and decide where to store the footage. It doesn't appear to have a backup mode but would store based on Quota or Group - you could direct ALL the footage to the Synology, or add it to the existing pool of disks but I don't see a true backup option that copies existing footage
If you're just looking to use two Synology and not do backups you could add them both in, setup a group with both in it and it should write to each, I'm not sure of the exact sequence it will use them though.
For backups - you have mirrored NVR disks normally so there is no risk if one dies you loose anything but if you can get footage onto a single Synology first, then there you should be able to snap/backup the footage to a second Synology using it's own interface, rather than Hikvisions if that helps? Most NAS units let you snap & replicate data on it somewhere else, that's how I'd try setting it up. Sorry for long answer, interesting use case!