Tripod - Your camera needs to be kept perfectly still to ensure your images line-up correctly.
Editing Software - You need to blend the images together once you have finished.
Plain Background - Not essential but will help make the blending process easier.
If your camera has it you can use the in-built multiple exposure setting but we are going to be taking individual shots and using Photoshop to blend them all together. This way we won't get the 'ghosting' you sometimes see when using the camera's multiple exposure option.
You need to make sure your camera is kept perfectly still as the slightest bit of movement will mean your photos don't line-up when you get them into Photoshop so clamp your camera on a tripod and leave it.
As we were working on a plain background auto focus and white balance worked fine but if you're shooting on a busier backdrop you may need to focus and set the white balance manually to ensure it's the same for each shot you take. Make sure you expose from your subject then simply direct them to the different places of your scene. If you're working in a small space you may find floor markers are useful as it's quite easy to end up taking a series of shots where arms, fingers or legs end up over-lapping.
There are a few methods you can use for the next step, however as we used a plain background we found the following method quick and easy to complete:
Open your images up in Photoshop, cut your subject out of all but one of the scenes (you need the background on one for the final image) and then drag all the other portraits you are using onto individual layers in the file you left the background on. As we only have two image we just have two layers.
Reducing the opacity of the top layer can help you line the two, or however many images you are using, up correctly. Just remember to bring the opacity of the layer back to 100% before you save your image. Once everything is in place, change the layer blend mode to Multiply and as a result, your portraits will now appear to be next to each other.
If you shot your portraits on a white background you simply have to drag both images into one document on individual layers, reduce the opacity of the top layer so you can double-check everything is lined up on each of the shots then change the layer blend mode to Multiply and as a result, your portraits will now appear to be next to each other. Remember to bring the opacity of the layer back to 100% then save your image.
For more tips on how you can blend two images together, take a look at this tutorial: Blending Two Images Together