There is also a cut path option that can be used. Just draw a line through your design, select the line and the design then select Cut Path from the Path menu. This method will break the design apart so that's kind of weird. You could also use the division tool, which does about the same thing as the cut path.
Skarekrow's method would be easier/better for a complex design. One step I'd change in his procedures, rather than moving the box after duplicating it, leave it and use the difference tool, and then the intersection tool. That way you don't have to worry about getting your shape lined up exactly.