If I use foam insulation on the outside (2") and in the 2 x 6 walls (from 4 - 5"), do I need to fill in the leftover space on the inside with fiberglass or Roxul, or just put the vapor barrier over it leaving 1 - 1 1/2" air space?
The space does not have to be filled but should not be between the two foam layers. The required R value does have to be met. To find your total R value: add the specified ratings for both foams. Why are you putting the vapor barrier on the inside? Is this code compliant where you are?