It is called double exposure.

It can be done in different moments in different ways.

I. During the taking of the photo.

• Some cameras have a switch that allows the "cranking" of the shutter button but does not advance the film. This way you could expose again the same frame.

On the big circle is the film advance lever. On the small circle, it is a small nudge, that prevented the advancement of the film but still prepared the mechanism for the next shot when using the main lever. Nikon FM.

• If you simply expose the film with the girl switching places you will have an overexposed image and a ghostly girl in both frames because the background is also exposed, so you need a device to mask two zones. This could be a black cardboard box with a movable window.
• I believe I saw some attachments that you could put in front of your lens, like a lens hood. In fact you can prepare a lens hood for this effect with two barn doors. It could be an interesting DIY project.

The advantage is that, if done right you could send your film to be developed and printed anywhere. It could be done in slide film.

To control the blurriness of the exposures you need to change the distance to the front of the lens, and the aperture. The closer and the wider the opening, the blurrier.

• Remember to use a tripod.

II. When exposing the paper.

• Take two photos using a tripod.
• During the paper exposure, you use the black box with a window and expose the two parts in two different stages. The reason for using a box is the same as controlling the distance of the first case. You have some separation against the paper you have a small transition zone that fades the two photos.
• Develop the paper.

One problem is that you do not have much control on how to align the negative vs the prior exposure. You could mark some reference points before exposing the paper.

It is only done in one copy of the paper. It also means that a lot of people could only make them in a black-and-white copy.

III. Just print two photos and cut and paste

• Use a sharp blade and some glue.

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https://es.wikipedia.org/wiki/Nikon_FM#/media/Archivo:Nikon_FM_(top_view).jpg