So you’re going to build an already-small superinsulated “pretty good house” (look it up). And you’re thinking of doing double-stud walls. Just remember that the extra 6″ or so adds up. It’s on both sides of the house, so it reduces the usable space by 1 foot in each dimension — x and y.
Let’s say you are building a boring rectangle that is 24×32 (inside dimensions). Well, that’s a 768 sqft. If you had instead built 25×33 (with single stud walls) then that is 825 sqft interior. In other words, you are giving up 57 square feet with your insulation. And that’s 7% of 825. So your house is now 7% smaller!!! Just be aware.
Instead you could do:
1) larsen truss
2) 4″ of polyiso added to the outside.
These are probably both harder than double-stud walls, I am just commenting that you should compare apples-to-apples — same interior square feet — when comparing options for how to build.