Perhaps there's only one door, just labelled "Door #1" and "Door #2" on either side of the door. And maybe there could be some kind of weird wormhole situation going on here where if you open the door one way you can go through to some kind of alternate universe (let's call this Door #1, although it's really just the side of the door labelled "Door #1"), but if you open it the other way you just go through to the other side. Or maybe there's two alternate univi and opening the door either way would give you access to paralell dimensions where opening the Door #1 would cause the Door #2 (the other side of Door #1 in the first universe) to be opened. By opening the door into the alternate universe, you're actually opening another door in that universe, just the other way, so who's to say that it's not Door #2 in the other universe you were opening, thereby opening Door #1 twice (as it's the other side of Door #1, but in another universe, and since Door #2 in another universe opens into another alternate reality, might be opening a whole new can of worms there (and this is only Door #1 (or the side of the door labelled "Door #1" (in our universe (assuming that we can lay claim to this universe and it's not really someone else's) that we've just opened)!). And god knows what would happen if the can of worms opened into the same universe that Door #1 (or #2, depending on which universe you're in and which side you're on) went into.