“ For redundancy, I am backing up my computers to 2 separate network storage devices.”

Hopefully he understands that this is not a true backup....he’s basically created a more complex RAID 1 running two NAS...

For example a flood, a House fire, theft, or a power surge is probably going to lose/fry both then everything is gone. This is a perfect example of thinking you have something backed up but if you store it in the same place you just created a more complex RAID 1...

If you want a backup you need to store a copy in a different location preferably a different region because if you put it at your friends house down the road they are also likely to be hit by the same disaster (i.e flood, tornado, hurricane, war, etc.) as you are.