Simulating this is similar to simulating looking through a window, having the map wrap around on the edges, and other such things. For any of these, including what you ask about, you could have dummy objects which are clones of the ones you want to see, and have these objects in the other location on the map that you want to see them on, and have any actions taken by the real object also affect the visual effect clone.

For example, for your 5-tile-long stairway you mentioned, let's assume you're on the top floor and someone else is on the bottom-most tile of the stairs and therefor on another part of the map you cannot see. If the stairs on floor1 extend from x=5 to x=9 (it's horizontal) then the lower person is at x=5. Let's assume this is z=1 for the lower floor and z=2 for the upper. Put a dummy clone (an identical object to the player) of the floor1 player on (x=5,z=2).

If the player steps to the east, moving the player up the stairs, they're still on z=1, so move the clone over a step too. To the person on floor 2 it looks like the player is seen on that floor as well. If the floor 1 player moves west from x=5 or east from x=6, or otherwise exits the view that you would see from the second floor, have the cloned object also take a step out of view so it looks like it's leaving, then delete it.

A better method would be to make a multilayered building using, invisibility and height/layer of the object, basicly when you move on the stairs you aither icnrease your height or decrease, based on the height your visual for the invisiblility sight also changes. Also, you have to make the mob only being able to interact with the objects on your current height and so forth, you wouldn't bump into an object wich is on a lower height level than you, would you? This also avoids you having to make multiple z levels for this, and by this you can also avoid having to make "dummy" mobs and you'll be able to see below your current level, however one major flaw is that opacity can't be accociated, by the heights, except when you will make a opacity system of your own, calculating where you CAN see, and where you can't.. and then using image objects to black out those areas. This basicly.. works very similar to my isometric engine and such. Not that this is very difficult, but I wouldn't really imagine you doing this by the method currently described. I ain't good at explanations so, I wouldn't find it difficult to believe anyone got what I said just now.

Best of Luck!