I decided to stick to the safest approach to make copies of my backups. In other words, my Mac backs up everything to one external drive and then I copy that entire backup directory to the second external drive.

I researched and found this command to be good for any files to be moved and synchronized from source to destination directories:

rsync -av --delete /Volume/Drive1/MyPictures/ /Volume/Drive1/MyPictures

That worked pretty well for any directory I store on the external hard drive except for Mac's backup directories. Running this causes an error:

rsync -av --delete /Volume/Drive1/Backups.backupdb/ /Volume/Drive1/Backups.backupdb

failed: Operation not permitted (1)
rsync error: some files could not be transferred (code 23)

Any suggestions how to address this? I tried rsync -rtb and got the same error. Copying manually using Finder also alerts with The operation can’t be completed because backup items can’t be modified.