Joist Generation

I must be doing something wrong. When i create a joist area and insert my horizontal joists, everything is fine. If I create my 2nd joist area, I can create vertical lines to represent the joists. The problem occurs if the two joist areas touch they become one area and then I can do either horizontal or vertical but not both. If the areas don't touch, they don't Merge. Is there any way to cut this merge off?