Double punching is very useful. By using one punch twice or two punches you can create a new shape. This method can be used in a few different ways. I will use examples from my October 09 Punch art designs
To make a smaller sqaure with a larger punch or as a first step to create a photo corner or bent arm punch a corner off with a squareStep 2 Slide the piece further in evenly and punch to form an L shapeIf the piece is big enough that you can hold it you can repunch it to make a new shape. For example punching an oval and then repunching little nibbles out of the edges to create a holly leave shape or pumpkin lid shape I used for the lid of Pumpkin Boy
The Daffyscrapper 