In a passive sky anchor, the superpressure balloon is sealed, while in an active system, its pressure can be varied.
Both versions have been tested in flight, but have had frequent failures with only occasional successful outcomes.
The tandem arrangement makes launching difficult, and this complexity can lead to mission failure.
The tandem balloon system is intended to increase the flight time of the zero-pressure balloon by damping the diurnal altitude variations caused by solar heating of the lifting gas and its subsequent expansion.
Likewise as they rise, the system displaces less air mass, limiting the ascent to prevent the lifting gas from expanding too far and escaping from the zero pressure balloon.