I would think that opposite pitches on the fan blades would cause water to be sucked in the side and forced out both ends, as Jim describes. Basic turbo-charger technology: One fan spins the other. It would be clearer how it worked if they were separate chambers with separate intakes and exhausts connected solely by the shaft. But, if the pitches are correct, it should work with the single intake.

However, I'm not not sure why this is better or more practical than just using the return.

Carl