One common theme is secrecy. Characters in these novels often go to great lengths to hide their other lives. For example, in 'Dr. Jekyll and Mr. Hyde', Dr. Jekyll has to keep the existence of Mr. Hyde a secret from everyone around him.
Identity confusion is also a key theme. When living a double life, the characters often lose a sense of who they really are. They may start to question which version of themselves is the true self. In 'The Great Gatsby', Gatsby has created a new identity for himself to win Daisy, but he is constantly haunted by his past and the question of whether his new identity is real or just a means to an end.
One common theme is identity. Since it's about double life, characters (either trainers or pokemon) often struggle with who they really are. Another theme is secrecy. There's always something hidden about their second life. For example, a pokemon might be a powerful guardian in its other life but just a normal pokemon in the eyes of its trainer.
One common theme is the contrast between two different worlds. For example, in many novels, there is a mortal world and a magical or martial world. The character has to navigate between the normalcy of the mortal world and the excitement and danger of the other world.
One common theme could be deception. The billionaire husband leading a double life implies that he is deceiving his wife or other people around him. Another theme might be power imbalance. His billionaire status gives him a certain power that he may misuse in his double life. Also, there could be a theme of discovery, like how the truth about his double life is gradually uncovered.
One common theme is loyalty. In a double agent novel, the main character has to deal with divided loyalties between two opposing sides. Another theme could be deception, as the double agent has to constantly deceive others to maintain their cover.
One of the main themes is identity. The two Veroniques, though living separate lives, seem to be connected in a profound way, making the reader question what truly defines a person's identity. Another theme could be fate. Their parallel existences suggest that there are forces at work that are beyond their control.
One possible theme could be identity. The heiress is likely to struggle with who she really is when living two such different lives.
Love and relationships are common themes. Just like in the original Twilight, the fanfictions often explore the complex love between different characters. For example, how they overcome obstacles in their relationships.
One common theme could be identity. Since there's a 'double' aspect, it might explore how characters deal with having two different identities or versions of themselves. Another could be duality, like good vs. evil if the double represents two opposing moral stances.
One common theme is deception. The use of a body double is often to mislead other characters, whether it's for personal gain, revenge or to avoid danger. For example, a character might use a body double to escape from an assassin.
One common theme could be friendship. Since it's 'double trouble', it might involve two characters getting into various situations together and their bond growing through it.