Superman gets turned into two Supermen in Action 222 (Nov. 56), in a story by Otto Binder and Al Plastino.
Superman is ground zero at a “q-bomb” test, which splits him into two beings.
They are virtually identical, the only difference being that only one has telescopic vision, while the other has x-ray vision. Off of this, they name themselves Superman-T and Superman-X. It’s an interesting situation, and neither one is the “real” Superman over the other.
The story doesn’t really delve into the possibilities of there being two Supermen much, content to use it to divert Lois Lane’s eternal suspicion that Superman is Clark Kent.
As the story reaches the ending, Superman-T, who has noticed something odd happening to his skin, steals a golden idol. Superman-X thinks that his twin has turned to crime, and tries to stop him.
In actuality, Superman-T has figured out a way to sacrifice himself into a meteor, which returns whatever Superman-ness he had to Superman-X, who becomes just Superman again.
This story would be reworked in a number of later tales, from Superman-Red/Superman-Blue, to the Sand Superman, and the Superman split caused by Satanis and Syrene, among others.