I need to make something similar to the poster on this site.
Does anyone have any references on how can I go about achieving something of this kind in Adobe Photoshop or GIMP?
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I guess each person is on one layer. Whichever is in front has a "well-designed" alpha channel: Where the background person is to show, it is fully transparent, whereas it is opaque where it should appear in front.
Two layers, the top one has "lighten" blending mode (blending modes is in the dropbox on layers panel). You need the background to be black. Try to experiment with different blending modes.