Couldn’t find a video of someone explaining this but i will give it a try:
You’ll keep the one skybox and make brushes with the nodraw texture that will act as invisible walls containing the player into the playable area, then between the building and the “open space area” you make a brush with the hint texture in the supposed path between the open area and the building area.
The hint brush should not be seen from the open area othewise the building will be rendered.
Maybe this image explains the solution a bit better than my text:
I can however be completely wrong and if anybody wants to correct me feel free to do so.