Two step stamping is basically layering two or more ink colors ... lightest to darkest.
Terrific Tulips is a perfect set to use as an example. If you look
here at the index page, you'll see different sized flowers. What you would do is stamp the larger image (your base) in a light color. Next you would use the smaller matching image and stamp over the base in a darker color.
In this example, the creator used Apricot Appeal and Pumpkin Pie to achieve the dimensional look of the tulips. She also used two different shades of green for the stem and leaves.
Hope this helps.