Luthor returns in World’s Finest 126 (June 1962), as Herron, Mooney and Moldoff have his latest invention produce unintended consequences.
A ray that Luthor builds to kill Superman in fact splits him into two beings, one is himself, and the other a negative version. The negative Superman is evil, and the two spend much of the issue fighting against each other. Batman and Robin get involved, but this is really a Superman story all the way. The “positive” Superman has an energy barrier around him that prevents the two from being able to make contact.
Batman theorizes that kryptonite will make the negative Superman more powerful – enough that the negative Superman will be able to break through the energy barrier. When the two touch, they merge, and Superman is restored to himself, and able to take down Luthor.