Christina,
I am not familiar with Lightroom, but I believe it does not support layers. If that is the case, the method I utilized would not work.
The photographs were processed using Photoshop CS4. The process was roughly as follows:
1. In Adobe Bridge, the two photographs (background and portrait) were selected and loaded into Photoshop layers (in the menu bar above, select Tools > Photoshop > Load into Layers. This opens Photoshop.
2. The top layer should be the portrait. Opacity for the layer is set at 50%.
3. In the menu bar above, select Image > Canvas Size. Increase the canvas size, so that you have free space around the images. This way, using the move tool, you can move the top image relative to the background, to place it where you need it.
4. With the top layer selected, click on the mask icon in the layers palette. This will create a mask for the layer. Click on the mask, and then select the paintbrush from the tool bar. Paint on the mask using black in those places where you only want the background layer to show (using my photos as an example, everywhere except the face). I used a fuzzy brush to avoid hard edges. If you erase too much, you can always paint the mask back by switching the brush from black to white.
5. Crop the final photograph to size. Save as tiff if you want to retain the layers in order to do some fine tuning later. Saving as JPEG will flatten the image and you will lose the layers.
It's actually easier to do than it is to describe the process. However, you do need a program with layers and masks capabilities. I believe that the latest version of Photoshop Elements (9) has full layer and masks capabilities, but earlier versions had limited masking capabilities.
Hope this helps.
Antonio