In one particular double-wall design, the  inner wall rests on the floor platform, as usual, but the outer wall rests on the sill. This allows insulation between the joist header and sheathing, to avoid thermal bridging by the floor joists. The double-wall can be assembled on the floor platform, with the tops of the two wall frames separated by the plywood plate at the top and by temporaries near the bottom. The assembled wall section is then raised to vertical using wall jacks. Obviously, the bottom of the outer wall frame will be lower than that of the inner wall by the height of the floor joists and subfloor.

The question that comes to mind is what the sill will consist of. As far as I can tell, without having a set of code books on hand, is that the sill may be a single layer of 2x PT wood, set on sill sealer and anchored in conformance with code to the concrete foundation. Width must be at least that of the wall frame mounted on it. In the case of the double wall described above, the sill would be 2×10, flush with the outer face of the foundation wall, or 2×12, overhanging so as to cover the top edge of exterior rigid foundation insulation.

However, I commonly see sills built up of a double layer of 2x. What is the requirement for thickness of the sill? What is the reason for a double layer? In the double-wall described above, the bottom plate of the outer wall actually will constitute a second layer of sill, although not bolted to the concrete with the first layer, and the floor framing would rest on a single layer.