What does it mean to double license code? I can't just put both licenses in the source files. That would mean that I mandate users to follow the rules of both of them, but the licenses will probably be contradictory (otherwise there'd be no reason to double license).

I guess this is something like in cryptographic chaining, cipher = crypt_2(crypt_1(clear)) (generally) means, that cipher is neither the output of crypt_2 on clear nor the output of crypt_1 on clear. It's the output of the composition.

Likewise, in double-licensing, in reality my code has one license, it's just that this new license says please follow all of the rules of license1, or all of the rules of license2, and you are hereby granted the right to redistribute this application under this "double" license, license1 or license2, or any license under which license1 or license2 allow you to redistribute this software, in which case you shall replace the relevant licensing information in this application with that of the new license.

(Does this mean that before someone may use the app under license1, he has to perform the operation of redistributing to self? How would he document the fact that he did that operation?)

Am I correct. What LICENSE file and what text to put in the source files would I need if I wanted to double license on, for the sake of example, Apachev2 and GPLv3 ?