Could you give me an example of this?
Gah, I wish I knew how to, I'm not that good at ASCii...
well, I'll give it a try
_______________
|._____________..|
|.|........................|.|
|.|........................|.|
|.|........................|.|
|.|____________|.|
|______________.|
So say the larger rectangle is the floor of the second story, the inner rectangle would be made of, say, 1x4 plates on the bottom of the second-story floor. Then, the top of the wall (covered in smooth plates of course!) could rest in the space between the larger and smaller rectangles. It actually results in a nice and snug fit that keeps the stories perfectly aligned, and sturdy. I'm sorry if I didn't explain it well, tell me if you understood that, and if you didn't I'll try a different way (I'm not that good at explaining things :/