Source - http://www.nwprophoto.com/tempint/uf.jpg

It is like a large beauty light. There are two drawbacks to this setup IMHO. Fist, it must be loosing a lot of light in reflections between the two umbrellas. Second, as any beauty light it would have a sweet spot where light would look just great and ugly anywhere else.

It It might be interesting to see how it would work with a shoot-through small umbrella. The light pattern would be exactly opposite to a Westcott brolly that has transparent plastic in the senter and white diffuser around it. It creates a hot spot of direct light in the middle surrounded by a soft hallo. This setup would make diffused light spot in the center surrounded by a hot ring of light.  As I picture it in my head, it must be rather ugly, but it is interesting to try anyway.