Deliberate mistake: Before the school bus crashes, a brown car is seen coming. When the angle changes we still see a brown car but it's a totally different model.
Deliberate mistake: Before Superman traps the black missile, its beam is really big and covers half of the lettering on the side. When Superman is behind, the fire is much smaller and the lettering is visible, allowing us to see Superman.
Deliberate mistake: At Lois' penthouse, before Superman says goodbye, the image is sharp and clear. Cuts to a close-up, and then check the next wide angle where he says goodnight and flies away: It's blurry and grainy because it's not live, but a screen with a previously shot scene to allow for the camera to turn left and, in a matter of seconds, no cuts at all, show Superman magically and quickly transformed into Clark Kent.
Answer: There are generally two methods of time travel. Skip vs Slide. When you slide through time, you are in essence rewinding or fast forwarding a tape. Time will accelerate to the desired moment. This method, the traveler will witness the rewind and will only allow one of them to exist. When skipping, you are plucking yourself from the time stream and placing yourself in the desired moment. This method, travel is instantaneous and can allow for multiples of the traveler to exist at once. Superman rewound time. He used the slide method and went directly to Lois after doing so. This means those he previously saved...died after his reversal.
Or he could have used the skip method. Like you said, it enables a traveler to pluck themself from the time stream and placing them at the desired moment allowing for two Supermen to be able to prevent both missiles from reaching their destinations.
He couldn't have used the skip method if he rewound time.
Bishop73