NOTE: Since this was written in 2018 cryptsetup-reencrypt appeared which should be used instead of luksipc, although it works on the same principle - downsizing partition and using free space to re-encrypt
PGP allows you to encrypt data using multiple keys, but any key there will be able to decrypt the data. What if you want to encrypt your data so that ALL keys are required to decrypt the data? For this you can chain encryption. Here is an example on how to chain three AES256 keys.