Using a key to unlock a door on 2 separate levels.

[Cyfiawn]

Hello,

I have 2 maps, with 2 doorways to each other (Upper and Lower).
Both doors start locked.
The player unlocks the Upper door first and goes through the door.
Then in the second map, they get another key to unlock the Lower door.
The problem is that when the player unlocks the Lower door from Map 2 side and returns to Map 1, the door is still locked from the Map 1 side. How can I unlock a door from both sides on the 2 separate maps?


Obviously, I can have the Upper door on Map 2 unlocked from the start, as the only way of getting to that door from that side is by unlocking it from the other side.

Here is an image to show the layout: http://i.imgur.com/SE2wE.jpg

Thanks,
Cyfiawn

PS. Sorry, I find it hard to explain this well, hopefully you get the idea.