If you only need to perform a simple file backup, I do not recommend using Duple Backup’s Hybrid Mode, as file encryption in this mode can reduce backup efficiency.
Alternatively, you can consider using Centralized Backup: install the CB on NAS 2, set NAS 1 as a file server by enabling SMB or rsync file services, and then back up the files from NAS 1 to NAS 2. This approach should also meet your needs.