» Tue May 15, 2012 11:54 pm
I believe a power attack is when you hold down the attack button/trigger. So if you hold down both triggers/mouse buttons at the same time, you will do a "dual power attack", i.e. a power attack with both weapons at once, probably with its own cool animation. If you just hold down one button, you will do a single power attack. So the bonus for a dual attack and a single attack won't stack, since you are either doing a dual attack or a single attack at any moment, but never both at the same time, if that makes sense.
As for your other question, you only need to fill one branch to get to a top tier Perk. Example, in Smithing, you can go Steel > Elven > Advanced > Glass > Dragon ... or Steel > Dwarven > Orcish > Ebony > Daedric > Dragon. Either way you get to Dragon; you don't have to do both.