We have all stacked layers in photoshop. It is something many photographers do on a day to day basis. Not Laina Briedis. She stacks 35mm film negatives. This is a technique not seen often in the film world and it creates amazing images that look like they had to of been done in photoshop.
Here is a little snippet about film stacking from Laina:
"There are a few ways to stack negatives to create interesting multiple exposure-like images. If you have a darkroom available for use, you can expose two negative/negative strips at the same time on the same enlarger (by stacking the negatives one on top of another), and with a bit of dodging/burning you should be set.
For a similar effect using a slightly different darkroom approach, you can also expose the two negatives/negative strips separately onto the same piece of light sensitive darkroom paper. If you prefer a digital approach or don’t have a darkroom at your disposal, you can stack two negative strips one of top of another and using a flatbed negative scanner achieve some pretty cool negative-stacked exposures.
If you’re good with Photoshop and would like even more control over which parts of your photographs are more prominent, you can even use two individually scanned negatives as separate layers. By erasing parts of one image, you allow the other image to show through more (creating a stacked-negative look). This I suppose would be like the digital equivalent of dodging and burning."
Via - PetaPixel.com