I do not believe that this is correct. It just does not fall into accordance with Chinese philosophies.
If kung fu is well rounded and is based on Taoist yinyang principle then there would be many combinations. yang yang or both standing( greater yang ), yin yang one grounded and one standing ( lesser yin), yang yin one standing and one grounded (lesser yang ), or yin yin both grounded (greater yin).
This is the greater and lesser aspects of yinyang principle.
The combinations of yin yang or yang yin would be determined by which of the fighters was the weaker or stronger one on the ground or standing up.