Ideally it would be a 2-disk solution with automatic back-up between the two. Android and Apple apps for access would be good too, as would spotify and iTunes support.
It’s worth bearing in mind that most two disk solutions will use RAID1, to ‘backup’ between the two. This is not a backup. What this does read and write to both disk simultaneously, meaning should one disk fail then the other retains a copy of your data. This not not a backup, it’s data redundancy. Should you, a family member, or anything malicious delete a file from the NAS you cannot simply restore it, and thus it is not a backup.
You may have already know this, I don’t know, but I think it is a point worth clarifying on. A real backup should be off-site, too.