Map over Map?

[Acruno]

I'm not sure how to fully describe this, but I'll try.

I was wondering how it would be possible (and if it would be wise) to do the following:

This is map A.

|AAAAAA|
|AAAAAA| A = Map A Tile
|AAAAAA|

Just a normal field or whatever.

Then this is map B.

|000000|
|000000| B = Map B Tile 0 = Blank
|00BB00|

For purpose of example, Map B is a balcony.

Now, if you were on Map A, you would see this:

|AAAAAA|
|AAAAAA|
|AAAAAA|

But if you were on Map B, you would see this:

|AAAAAA|
|AAAAAA|
|AABBAA|

This would include people. I.e. Map B players would be able to see Map A players, but Map A players wouldn't.

How could this be achieved?