@lyst83: I tried making a regular duplex and it won't work for me either. Looks like you're going to have to make a lobby or enclosed porch of some sort, mark it as Public, and then set the door as NPC. The enclosed part is very important! You can use an archway, but it does have to be walled in.
If that does not work for you (aesthetically I mean), you'll just have to mark the other side of your house as hidden and pretend an NPC lives there.
@Hosfac: The high rise shells are available in buydebug. There are several that fit very comfortably in 30x30 lots. Once you set them down, however, you cannot move them again so make sure you put them in the right place (like, if you want to have parking spaces, make sure you place them or leave enough room before you put down the building). I suggest looking at the buildings in BP before you start messing with creating one of your own. They're pretty tricky, especially the walls and windows. You'll notice on the EA high rises, the "outside" walls and windows don't actually show when you have the roof up (or are on a higher floor) -- you only see the shell building. Pay attention to how they do the lobbies as well. There is the outside shell building door, and then they use a walkway to get to the actual in-game lobby doors. That is to say, there are two sets of "main" doors: one to get inside the shell, and another to get to the actual playable lobby.
So to answer your question, yes you can move the high rises around, but not after they've been placed.