It is a cardinal rule that nothing can be in more than one place at the same time, but rules were made to be broken, and an exception is allowed for a special kind of thing called a "backdrop". For instance:
The Upper Cave is above the Rock Pool.
The stream is a backdrop. It is in the Upper Cave and the Rock Pool.
Backdrops are ordinarily in the background: if the sky needed to be referred to in the course of play, it might be represented by a backdrop, for instance. Here we have a stream of water running through two rooms, though it might be any number. Backdrops are always fixed in place.
Backdrops can be put in regions as well as rooms, and if so, then they are present at every room in the given region (or regions), as well as any specific rooms they may also be put into. For instance:
The Outdoors Area is a region. The Moon is a backdrop. The Moon is in the Outdoors Area. The Moon is in the Skylight Room.
The special place "everywhere" can be given as the location of a backdrop to make it omnipresent:
The sky is a backdrop. The sky is everywhere.
Inform assumes that backdrops are also scenery unless told otherwise, so this will not result in messages like "You can also see the sky here." being included in room descriptions. In the case of the stream above, we could artfully mention it in passing in the room descriptions of the Upper Cave and the Rock Pool.
See Moving backdrops for ways to place backdrops in dynamically changing selections of rooms
Disenchantment Bay 3
Suppose we wanted to have the glacier visible from the Cabin of our boat, and anywhere else we might add to the game:
The view of the Malaspina glacier is a backdrop. It is everywhere. The description is "The Malaspina glacier covers much of the nearby slope, and -- beyond it -- an area as large as Rhode Island."