This is a neat little game with lots of potential! My favorite aspect is playing the level once in light and once in darkness, almost like a mini memory game; I like that it's a join across dimensions in a sense. The art is also beautifully well done.
My feedback is mostly the same as what others have pointed out below. On lives specifically, people have pointed out going backwards a level does not feel very great; as an alternative, maybe the Light character doesn't need lives at all, and the Dark character has three lives, which upon losing them means restarting the same level as the Light character.
Fun game! Hope the feedback is helpful.