Here I go, showing my age again...
Concentric masts with two rotators, exactly as described below, were used
by K6RR at his mountaintop multi-multi during the 70s to turn large
> Chris R. Burger, ZS6EZ, wrote:
> > A new idea that's being incorporated in my new station's design is
> > to use two concentric masts. The inner mast has a concentric
> > rotator, while the outer mast has a motorcycle drive sprocket to an
> > offset rotator. This way, you can have an independently rotatable
> > top antenna without much additional cost. The rest of the stack is
> > turned through the motorcycle chain.