Hi @javier-lopez-1s,
Thanks for your post and for being part of the Cesium community.
Unfortunately as you detailed rendering polygons with outlines on terrain is not supported. The issue you linked Support for polygon outlines on terrain · Issue #6694 · CesiumGS/cesium · GitHub is the most recent activity on exploring support for this feature, please feel free to comment on it with your use case or to express support for prioritizing it.
If you were looking to render multiple overlaid polygons, you would need to treat them the same as your example (using clamp to ground) to get the correct rendering effect you described.
If you let us know more about the design you are trying to achieve, we can try to help further and potentially find alternative solutions.
Best,
Luke