You could perhaps do it by using an enormous tie beam. I don't quite see why it's an issue when that middle interior post would abut the two story interior middle post.

By my eye it looks like your stairs are pretty steep. Code in Mass is 10" min tread, and 8.25" min riser. And you need a 3' landing at the top.

What town are you in? I live in Becket, and design and build timber frames. I teach at Heartwood too, so I'll probably see you during the class.