Both Timeshift and Snapper use BTRFS snapshots as a means to retain the filesystem's state. Because BTRFS snapshots must be stored in the same filesystem which contains the subvolumes they are a snapshot of, snapshots cannot be used as true backups. By true backup I mean that the content is stored on a different, independent device. BTRFS filesystems can span multiple hard drives, but that's not a backup solution.
However, what you can do is use
btrfs-receive to backup the snapshots created by Timeshift or Snapper to a secondary device. Said device would need to have it's own independent BTRFS filesystem.
In short, no you cannot configure Timeshift nor Snapper to store "backups" on a separate device, but you can instead send those "backups" to a separate device.