It seems like I've done some reading about U-boats where there were two Enigma machines. I tried to search for information about this, but most of the time searching for double encryption brings up pages about the fatal mistake of encrypting the message key twice, not something to do with an entire message encrypted twice with different machine settings. So that is my question - were some messages literally encrypted twice as in plaintext -> ciphertext1, then ciphertext1 -> ciphertext2 so that the U-boat radio operator might decrypt the first level and the message would be handed to the captain/officer who would then decrypt it again with different machine settings/key.
Did that happen for high security messages?
How did BP deal with such messages? Were they able to break them?