hi Jodi,
One potentially reasonable way to construct the hybrid double stud wall is this:
2 x 6 exterior structural wall with exterior structural fiberboard sheathing.
Insulate stud bays of 2 x 6 wall with batts or blown insulation.
Install Huber R-sheathing, 1 1/2", R-6 on the interior face of the 2 x 6 wall, for continuous insulation R-6 and seal it as the air barrier.
Install 2 x 4 interior wall directly against sealed face of zip sheathing. Run electrical and other mechanical needs. Insulate this wall with blown-in insulation - cellulose or fiberglass.
Gypsum board interior finish. No poly.
Test the walls with air barrier in place, before installing the interior 2 x 4 wall.
Make sense?
This way, you don't have to install a separate layer of rigid insulation between the two walls - the insulated zip sheathing does it for you. I did the math, and you are safe as far as condensation risk as long as the interior wall is 2 x 4, since the minimum insulation level you'd have outboard of the air barrier (at the studs) would be R-12.
Hope this helps.
Ciao,
Rachel