Source for photos:
blog.news-record.com/staff/culture/Culture Shock - The Joker-thumb.jpg
polivox.files.wordpress.com/2008/03/jack-nicholson_joker.jpg
Image Transformation:
For the Nicholson photo, I simply used the Brush Strokes --> Dark Strokes effect.
For the Ledger photo, I had to scale it to around the same size as Nicholson using Transform-->Scale. I then applied the same Dark Strokes Filter.
I created a New Image with a black background and put each doctored image in a different layer. On the top layer, the Ledger photo, I created a Layer Mask and used the normal gradient tool to show an overlap in the middle. That's it.