How can I build houses on top of each other? I've seen people do it but the wiki says houses can't share doors or wooden platforms. What is the correct way to build multi story houses?
The generally accepted way is to stack rooms on top of each other, with doors leading out into a common vertical shaft of some kind.
Put the wooden platform floors out in the common areas so you can get around between the various rooms.
You want something similar to this:
Houses cannot share single tile walls either but they can share floor/ceilings as seen above.