While not as elegant as a full RAID solution, using a second NAS to back up the first provides two key advantages for small networks.
The most important advantage is that it provides complete redundancy. Since each NAS is self-contained with its own drive, power supply and processor, it stays up when anything—not just a drive—fails on the other.
Using a separate, networked NAS also allows physical separation between the primary and backup NAS. You can use this advantage to place the backup NAS in a different, perhaps hidden, location to provide additional physical security. You can even use a wireless (in a pinch) or powerline (preferred) bridge if you don't have an Ethernet drop where you want it. Since the backup is a background activity, you don't need the best in throughput.
In order to make this approach work, one of the two NASes must support scheduled backup to or from a networked drive. Most all NASes support backup to a USB-attached drive and many can do this trick with a networked share. But some drives support only attached-drive backup. So you may need to download the user guide of a product you're thinking of purchasing in order to know for sure.
The separate backup NAS approach can also be less expensive than RAID
A possible disadvantage is that the backup may not be an exact duplicate of the primary due to the file changes that occurred between backups. But depending on how you use the primary NAS, the effect can range from the loss of a few hours work to no data loss at all.
