I have 2 usb sticks, 8GB each:

usb1 = /dev/sdb

usb2 = /dev/sdc

usb1 is a full dm-crypted drive and I am currently using usb2 as a backup, just incase usb1 fails. I am cloning usb1 to usb2 using the following command:

dd if=/dev/sdb of=/dev/sdc

This works as I have tested usb2 and everything seems intact.

What I don't like about this method is that it takes about an hour to run the dd command.

Once I have cloned usb1 once, is it possible to keep usb1 and usb2 in sync without having to clone the whole drive using dd each time a text file is changed?

Can I use something like rsync to sync 2 usb sticks which are fully dm-crypted drives?

I can dd usb1 to usb2 without decrypting usb1. I'm hoping a solution exists where I don't have to decrypt both usb1 and 2 to sync them. I'm hoping the sync program can figure out the differences at binary level?