If you wanted to make a Passageway warp from one side of a screen to the other side, the following problems occur in the Passageway screen:
* You always enter on the right side.
* Both ladders lead to the destination.
Hrmm.
A screen with a Passageway to itself
After some more investigation, the source of the problem reveals itself:
If Link wanted to leave a Passageway from the ladder he entered from, where will he end up? The answer is: the warp return square in the initial screen.
Which is identical to the warp return square in the destination screen, if both of those screens are equal.
Since I cannot change the answer to that question, I shall have to resign myself to this behaviour.