Power is a major theme. Two Lucifers would bring a lot of power into one story, and it can be about who has more power or how they use their power together.
In the world of 'lucifer meets lucifer fanfiction', a common plot could involve a clash of egos. When two such powerful and complex characters meet, there's bound to be conflict. Some stories may also explore a romantic or at least a deeply emotional connection between them, as they are essentially the same being in different forms, and this connection can be both fascinating and disturbing at the same time.
Lucifer and Satan were not the same person. Although Lucifer was called the Morning Star, it did not explicitly indicate that he was Satan. Satan was the name of the ancient snake, and Lucifer was an independent existence. Therefore, Lucifer Morgenstern was not Satan.
I'm not entirely sure specifically what 'lucifer fiction' refers to. It could potentially be a genre of fictional works that somehow involve the concept of Lucifer, perhaps in a religious, mythological, or even a more creative and fictionalized context.
Lucifer can be a good comic depending on your preferences. If you enjoy dark and mythical themes, it's likely to appeal to you. The character development is strong, and the world-building is immersive.