There is always an interface whether the layers are singly coated or coated in multiples at one time. There are tiny refractive index changes due to the chemicals present in the layers due to the fact that layers differ in content. The difference within the layer itself is substantial.

Think of the lens analogy. You have to coat lenses even though you may use the same glass in every element. It is the interface that counts even if there is no air there.

PE