Make a duplicate copy of backup folders

What is the easiest way to make a duplicate copy of my backup folders, so I can store them off site. I plan to just put them on a USB HD and re do them once every few months or so, limiting data loss in case of fire, flood, etc.
Or is it a better idea to simply copy the primary encrypted files to the off site HD?
Or is it a better idea to simply copy the primary encrypted files to the off site HD?