In earlier versions restic used a directory restic to put the snapshots in but I did not find a possibility to set it.
In NethServer one restic backup schedule is bound to one repository (storage backend). I think this was done to not complicate things.
If you can create buckets without big effort, I’d leave it like that. If not, we’ll find a solution.
EDIT:
Maybe it just works if you use the same password for both restic backups in /var/lib/nethserver/secret/restic_* instead of the autogenerated ones?
You may need to remove existing backups in the buckets first.