I never backup a backup. If the original backup has any type of error or issue, I would only be duplicating that and might end up with two unusable backups.
One option would be to use Time Machine for one backup and use Carbon Copy Cloner to make the second backup (from the Mac, not from the Time Machine external drive).
Another option would be to make two Time Machine backups as described in Use Time Machine on your Mac to back up to multiple disks - Apple Support
See the following excerpt from that support article:
For extra security and convenience, you can back up to multiple disks. If you work in two locations, you may want to keep backup disks in both places.
Time Machine rotates the backup schedule among the disks. For each disk, Time Machine backs up everything that’s changed since the last time that disk was used. If you designated items that shouldn’t be backed up, the list of excluded items applies to all backup disks.