I assume by a hidden wall, you mean to check the box that says hidden. After trying that and fiddling with the walls, I get the image below. I assume if I make a new wall type for this, I can get the double lines to only show single lines. But I'm still getting the line on the wall and the intersection of the overhead wall not very clean. Is there more that I can do here? Am I on the right track?
BTW - the "none" material is a a work-around to get the room sizes to round down. I find that when I put rooms in, it will either round up to the next inch or you can have it not round at all (I think). What I need is to show whole inches but I can't round up. I hope there is a better way to do this, but the only thing I could think of was to put a 1/4" thick material on the wall so that the room size always measures 1/2" less (1/4" per side of room). This gives me the effect I need, but sure isn't ideal.