One piece of Conservation Glass is acceptable, however the cover will fade ever so slightly over time. Two pieces of Conservation Glass do indeed block more than one piece of Conservation Glass, thus protecting the book even more.
Example: 100,000 rays of UV hit a piece of Conservation Glass. 1,000 rays make it through the glass. Then the 1,000 rays hit the second piece of Conservation Glass. 10 rays make it through the second piece of glass. This makes two pieces of glass 100x better than one. Of course, glass can never block 100%, but it can get closer and closer (99% for one piece to 99.9% for two pieces).