It is for instant duplication.
Note that this caption is incorrect, on the left is Kabo, played by Shinichi Nakano.
The explanation is that instead of filming with one camera and then duplicating the processed film later, they can film 8 cameras simultaneously and have 8 films duplicated on the spot.
Useful part of the transcript:
They complete about one film every day, and as you can see,
they've devised an ingeniously economical way to reproduce the film.
If Mr. Ogata and [Banteki] each strap four cameras together with duct tape
and push all the buttons on the cameras simultaneously,
they can instantly produce eight copies of the film,
which they then screen for small audiences, like this group of office workers.
This explains that when they start to shoot, they press the shoot button simultaneously on all cameras at the same time:
Array of Fujica Single 8 P1 movie cameras:
The P1 was one of the first Single 8 cameras released by Fuji in 1965 (and in various versions up to 1977). Single 8 was a film format introduced by Fujifilm in 1965 to rival the Kodak Super 8 format.
The single 8 cartridge had 50ft of 8mm film. The camera ran at 18fps, giving around 3 minutes and 45 seconds of footage.
You can see the cameras being fitted to a tray to keep them together here:
Second picture in OP's question:
On each side of the array is a 1963 Nikkorex 8F 10/1.8 8mm 16fps film camera.
The two in the middle are the earlier 1960 Nikkorex 8 film cameras.
The camera uses whats called a double 8mm film spool containing 50ft of double film.
Whilst this gives you 4 minutes of playback, at 16 fps, there is a catch:
The 50ft is split in two, running down in tandem on a reel thats 16mm wide.
So you shoot the first 25ft (2minutes), then you open up the side and take out the spool, flip it over, and reload it to film the second 25ft.
Obviously, filming a porno is a bit difficult if you only get 2 minutes a pop and have to reload so this gives you a longer continuous running time instead.
So originally, I thought that if assembled in this way, they can film around 8 minutes of film without reloading - ie. film camera 1, then 2, then 3, then 4.
But based on the information given in the youtube introduction video and the film footage that shows them pressing all shooting buttons simultaneously, its simply to duplicate on the spot.
I'd also like to double check if this is likely to be Shohei Imamura, or perhaps an actor in the film, being portrayed as filming pornography.
This is a scene from the film, with actors.
In the OP's picture, on the left is Shinichi Nakano, and on the right is Haruo Tanaka
Haruo Tanaka left, Shôichi Ozawa (playing the protagonist, Ogata) right.
I only see the Fujica Single 8 P1 movie camera used in the movie, I don't or cannot easily see the Nikkorex cameras at all...