plan comparison question

[small cabin dreamer]

I have been looking at both the 14x24 plans and the 20x30 1 1/2 story plans. Is it possible to start with the 14x24 (big enchilada or little house) and then add to it the 20x30 with 1 1/2 story or will the roofs not line up properly when it comes time to add the 20x30 perpendicularly? I want the tallest walls the plans will allow for both so I have useable space upstairs.