To answer question #1, In most cases your local building codes allow you to shingle over the old roof once. Although, here in Florida it is not recommended due to the increase in weight to the infrastructure in the case of a hurricane.
To answer question #2, Asphalt roofing comes in two types. Laminated shingles, also known as "architectural" or "dimensional" shingles, are layered, and their thickness and depth make them look more like slate or wood shakes. Three-tab asphalt shingles, though similarly priced, are made in a single layer.