I think any sort of big hp requirements these days can be met with by one turbo, but they might not work well down low, so some claim that even in a small displacement high hp application, two smaller turbo's reduce lag over one big turbo. Other's say that because of basic physical laws, the reduction in inertia and flow caused by splitting/dividing the exhaust energy in half outweighs the advantages of lighter, smaller components in a single turbo so one big turbo housing is more efficient then two smaller ones.
I remember seeing this at a shop:
I guess it's a choice of what kind of power delivery is required from the available powerband, but there is always going to be compromises.