I have a couple of Vixen files that have different TPI on each side. I think 8 or 9 on one side and 12 on the other. When used on sheet metal edges they are run parallel to the edge, not across it so the courser tooth will work fine. You can use it sort of like a wood plane on the edge. It will make straight edges if used properly and removes material fast. I use it to file to a trim line while keeping the line straight.
As others mentioned, they are used for fiberglass also. If you use any file on fiberGLASS, the glass will dull the teeth and ruin them for use on metal. I have one for fiberglass and one for aluminum. They do not work well with harder materials like steel or stainless steel.