Tested with Godot 4.2.1.
In a collision layer, you can paint a second collision area with the 'Add polygon tool'. However, in an occlusion layer adding a second collision area removes the first one.
To work around this, you can add a second occlusion layer:
- In TileSet -> Paint, paint the first occlusion area on the tile on Occlusion Layer 0
- Save the changes to the scene
- Under the TileMap node's properties, open the Tile Set properties
- Under Rendering, click Add Element
- In TileSet -> Paint, paint the second occlusion area on the tile on Occlusion Layer 1
- Save the changes to the scene