My GPG4USB tutorial briefly explains that: http://32yehzkk7jflf6r2.onion/gpg4usb/ "Lastly, it's a good idea to back up your private key. If you lose it, you won't be able to decrypt messages and they will effectively be lost forever. Right click on your key in the right panel and select "Show Key Details". You'll see the dialog above. Click "Export Private Key" and save the file in a secure location." The second step, not explained in the tutorial, is simply to copy the key to the other GPG4USB and import it the normal way.