If you plan on using all functionality on both NAS systems then getting the same model will make sense. But if you want to use a secondary NAS for remote backups then this would be overkill. I would get a simple J or value series NAS for backing up and would not even consider a RAID.
But if you plan that secondary NAS will become primary when the main NAS gets broken/stolen then this is a good plan.
I hope this helps.