Instead of using double exposures or printing from double negatives we now have the technology available to us to make these changes in post-production, allowing for quite astonishing results.OCA, Photography 2: Context and Narrative, p. 42
Use digital software such as Photoshop to create a composite image which visually appears to be a documentary photograph but which could never actually be.
To make a composite image you need to consider your idea and make the required amount of images to join together.
Upload the images and decide which image you’ll use as your main image and background. Use the magic wand to select sections of image from the others you wish to move into your background image. Copy via layer and drag into the background. Do this repeatedly until you have all the pieces of your puzzle in place. In order to make it more convincing, use the erase tool on each layer to keep the edges soft and to create a better illusion. Be aware of perspective and light and shadows for the most effective results. 
As an example, we are referred to the montage of Tony Blair taking a selfie with a pasted burning oilfield in the background, made in 2005 by Peter Kennard and Cat Phillipps . Given the nature my relationship with the Labour Party, this image has been close to my heart since I first saw it in Tate Modern a few years ago. I see it every day as a DIY fridge magnet, together with Richard Hamilton’s 2007 Shock and Awe  that was (I think) exhibited next to it.
[6Jan20] I have two initial ideas.
1. Two people on a backless bench, one facing forwards, the other backwards. Swap the heads in software.
2. Buy a fish, say 6-10 inches long. Photograph it on a wooden board. Swap the tail for a second, reversed head.
The problem with (1) is finding a bench with an approprate background and two volunteers. No. (2) should be easier, but would the scales match up?
[7Jan20] experimenting with stock fish shots, the difficulty arises with fish that differ top to bottom – I had envisaged them covered uniformly in scales. Check a fishmonger for anything suitable.
On the benches, head reversal front, at the risk of self plagiarism and cultural appropriation, I have deplayed an image from EyV Asg.5, Benches, an abandoned project option. Here’s before (fig. 3) and after (fig. 4).
At least that’s one in the bank in case the fish head notion doesn’t work but I’d prefer a custom image, ground level, two subjects, closer in.
[15Jan] Next I worked on an image taken on a largely empty train, returning from Southwark Cathedral.
This was not as successful as the first effort, largely because of complications around the neck and chin (fig. 6). This is a shame because leaving the reflection of the more distant head unchanged would have leant a nice sense of incongruity.
[17Jan] My final effort for this exercise is a bit of a cliche, but at least it works. I have been drawn to sardines lately after seeing images by Facchetti (fig. 7) and by Jagodic (fig. 8). The link of sardines and commuters is apparent and so, having to travel up to town for a 9 o’clock appointment this week, I photographed a sardine can in anticipation (it seems that cans with a key no longer exist, at least in the supermarkets I use) and took a camera intending to take a few shots of the throng before boarding. I didn’t realise how dark it still is at 7:30 and so have had to recycle and perspective-correct an image from EyV (fig. 9) as a temporary measure. I will try to remedy this in the summer.
1. Jones, J (2013) The Tony Blair ‘selfie’ Photo Op will have a place in history [online]. theguardian.com. Available from https://www.theguardian.com/artanddesign/2013/oct/15/tony-blair-selfie-photo-op-imperial-war-museum [Accessed 8 January 2020].
2. Spencer, C. (2014) Collage Master: Richard Hamilton at Tate Modern [online]. apollo-magazine.com. Available from https://www.apollo-magazine.com/richard-hamilton-tate-modern/ [Accessed 8 January 2020].