Software Recommendations to Mirror one External Hard Drive to another.

+1, use either Carbon Copy Cloner or SuperDuper! to clone your HD. Do it nightly to keep them as synchronized as possible. Both apps do the same thing, one is not better than the other but everyone has their own preference on which one they prefer. Both vendors will allow you to test their app prior to purchase so that gives you the opportunity to see which one YOU like.

Ok, on the top of my head, I can thing of at least two things that could work:

1. A hardware solution - Using an external drive enclosure (or better still, a dedicated NAS device) for these drives that supports RAID. One of the simplest RAID configurations would provide for drive mirroring.
2. A software solution - Although you could potentially write a script to have macOS mirror two drives, it would be far simpler to use an app, like Carbon Copy Cloner, for this purpose.

Another vote for CCC.:)

To not leave you "in the dark" about RAID:

RAID = Redundant Array of Independent Disks. It is a technology used to combine multiple drives for improved performance, redundancy, or both. The specific RAID version that allows for mirroring two drives is RAID 1. Most NAS (Network Attached Storage) devices support RAID at the hardware level. That means, it will work with just about any HDD. Alternately, not all external drive enclosures, support RAID either at the hardware or software level.

In RAID 1, data is written identically to two or more drives, meaning if one drive fails, the other has an exact copy, ensuring data redundancy. While RAID 1 offers fault tolerance, it doesn't improve performance or increase storage capacity, as each drive holds the same data. This setup is ideal for protecting important data against hardware failure.

No problem