I had the same issue with a door I saw, the other end not being created yet or something. So I just made my own door with a potion I had! It was cool since someone else also took the door after I did. :)
Say Door A is in East Spice. Door B is in West Spice. Doors can be almost anywhere, except houses... not sure about anywhere else. Doors can be on the same street, just a couple meters away from each other.
Going through a door will make you appear at the other door, whether it's in another street or not. You can easily escape back to your previous street by going through the door again.
Pouring a door potion will make the OLDEST door disappear and a new door appear at your house. So Door B is the oldest, the one in West Spice one will disappear, East Spice is now Door B, and wherever you are is now Door A.
I think that when you can't go through a door, it means that someone poured the potion but they didn't go through it.
And I really don't like how these door potions are linked. I wish that it was totally random every time. I realized they weren't random when I poured one and went to the location that I had poured one previously. I was in love with the potion till I realized that. =/