2. Split the colours
On the first image, go to Select>All and then Edit>Copy. Then, on your second image, go to Edit>Paste to paste the first in as a new layer. In the Layers palette, double-click on the topmost layer to open up the Layer Style window. In the Advanced Blending section you’ll see three channels, R, G and B. Uncheck the R (red) channel and you’ll see the main image turn red and blue. Click OK to the proceed.
3 Adjust the image
With your top layer still selected, press cmd + T (Mac), or ctrl + T (Windows) to adjust the scale, angle and position of your top layer. Position the layer with the most aesthetically pleasing results – scale it up or down in size (don't scale up too much or you’ll lose detail), reposition it, or right-click to access options such as flipping it horizontally or vertically. Once done, click on the tick button in the top menu bar.
4 Adjust the colours
You might find that the blue or the red is overpowering, or too bright, which can diminish detail. In the Layers palette, click on the Create new fill or adjustment layer button and select Hue/Saturation. Then, in the window that appears, select the Reds or Cyans channel from the drop-down menu. Use the middle Saturation slider to adjust the intensity of colour. You can change both colours using the Hue slider.
5 Advanced colour control
Further colour control is available by using a Selective Color adjustment layer. Add one in the same manner as the previous step. Once active, the Colors drop-down menu at the top allows you to target a specific hue, while the Cyan, Magenta, Yellow and Black sliders allow you to adjust it. It’s similar to using the Hue/Saturation controls in Step 4, but with more control over the exact colour.
6. Tweak contrast
You may find that your image needs some contrast adjustments. Select the layer that you wish to adjust in the Layers palette and click on Create new fill or adjustment layer again, selecting Levels this time. In the Levels window, click on the Clip to layer button at the bottom – now the levels adjustment will only affect the layer below. Tweak the controls until happy, and repeat for the other image layer, if required.