Multiple turbines have long been used in hydroelectric generating systems. I imagine that the principal difficulty encountered in wind-driven systems was weight, particularly the weight of the shaft which transmits the force from the propeller to the generator. The first illustration in the linked article bears this out--the return of useful energy must diminish significantly as the shaft is lengthened.

The development of lighter materials which have sufficient strength makes such a system more practical, but it may yet be many years before a really useful "twenty-five turbine" system is built.