If one of the top two cards of your library is already a double-faced card, put it into the new sleeve with either of its two faces facing outward. Only the outward face will be part of the new modal double-faced card, while the inward one ceases to exist for the rest of the game. For example, if the top two cards of your library are Brainstorm and the modal double-faced card Valki, God of Lies // Tibalt, Cosmic Imposter, you will end up with a modal double-faced card that has Brainstorm as one of its faces, and either Valki, God of Lies or Tibalt, Cosmic Imposter as the other face, with either face as the front (your choice).