1. Here we have pasted two separate photographs together. One is the background of the castle, the other is the man (who is on a layer above the image of the castle).
2. Select the layer with the background of the castle. Goto Filter > Artistic > Film Grain. Adjust the Grain Slider to the right a bit. Make sure the Highlight Area and Intensity sliders are all the way to the left (set to zero).
3. Repeat step 2 for the object/person layer.
4. The graininess should add noise to both layers and help blend the two photos, disguising the fact that they are not of the same photograph.