Actually you're a noob because you've never even heard of anyone double layering. Again, its quite common.
If you are considering double or triple layering, you have to ask yourself what your goal is. Is it for less road noise? If so, there are better materials to use such as closed cell foams. If its for added mass loading (stiffening the panel more), you should consider going with a single layer of a better quality or thicker material. Layering the mat stuff does add more and more mass, but mass loaders like that work better the better that adhere to the panel. Once you start getting multiple layers, you start having sagging problems with some cheaper brands, air bubbles become a real hassle, etc etc. Again, if you can acheive the same (or close) results using a single layer of a better material, you'll be better off.
That being said, as someone else above said, sometimes adding multiple layers in certain strategic areas can help. Im certainly not saying multiple layers do nothing, only discussing possible better options. For example, the roof of my blazer flexes very badly since I shaved the luggage rack. I could tack two or three layer of mat up there and hope it stays. But I dont necessarily need to add weight, just stiffen the roof. So I'll be putting up a layer of fibergrass (cloth soaked in resin), this should effectively stiffen the sheet metal. I'll then spray a layer of sound deadener over the top for a sound barrier, and be done. It will weigh considerably less than multiple layers of mat, will stay up forever, cost less, and be just as effective.
Just consider your goals (two main ones in sound deadening: mass loading/stiffening, and a noise barrier/absorber). There are many ways to skin a cat, and just adding more and more layers of sound deadening mat isn't always the best plan.