Does doubling insulation double the R value

Ok, now that we know where you are I would say I am not in total agreement with the proposal.

The placement of the metal skinned insulation up against the osb will undoubtedly leave a space that under certain situations could result in the formation of condensation from exterior temperature/humidity conditions. I would be uncomfortable with that possibility since it would be in a concealed cavity. When I say a space, I mean any space, even as little as an 1/8" would be detrimental to the situation.

I would be more inclined to go to one of either of these two approaches:

1). Install foam sheathing, (not metal skinned panels), over the exterior of the osb to give you 1/2 to 2/3 of the total proposed R value for the wall assembly. This will give you a continuous layer of insulation over the framing/sheathing and better performance than cavity insulation only. You should also be very conscious of detailing the edges of the foam at the perimeter of the construction so as not to allow an air flow between the foam and sheathing. If you want to add the additional insulation in the framing cavity you can use rock wool or fiberglass batts to give you the balance of the total proposed R-value.

2).Frame and sheath as you propose but have closed cell 2lb. density foam spray applied to the back of the sheathing at each stud cavity. Spray to the full proposed R-value and forget the fibrous insulation.