You know it's been studied at Harvard that two counter-rotating savonius mills placed very close together, with the power side of the rotors close to one another will cause the two rotars to "phase lock", and this combination will create more power than two individual rotors, due the
Magnus Effect.
The layout would be something like this:
In practical use this isn't really feasible, for the fact that most VAWT's take wind from multiple directions, and not just one prevailing wind direction. So a permanent layout of two VAWTs wouldn't always be placed in the optimum direction.
Though it's not something that seems easily usable in most cases, it's another 'effect' of counter rotation that most people don't take into consideration.