The plan:

• Subject looks off into distance or directly at the camera.
• Multiple exposures behind/to the side or infront.
• Centre of the frame.
• Need to line up eyes if they are both pictures are same angle.
• Main photo no expression, layered photo shows facial expression.
• Dark background, layered photos less or more transparency.

For this multi-exposure montage picture I imported my model into photoshop, duplicated it two more times and dragged the two copied pictures behind the main photo and selected both of them to have 50% opacity with no background and put them equal on each side.

For this photo I imported the same picture twice with both overlapping each other with the multiply effect on both. Then created a box on the left with the left photo, made it pink, flipped the other same photo to the other side but made it green, but left a thick white bar in the middle to show the split.