While that would certainly be possible, I’d suggest that you would lose some of the advantages of this design, and wouldn’t gain much.
It looks like the designer wants to incorporate a dead air space between the inner and outer barrels, otherwise it wouldn’t work to retain heat. The outer shell would thus have to be built freestyle (no form). While I recognize that this is more of a schematic, I’m thinking that the design also depends less on the insulation blanket to retain heat.
I think the biggest issue would be you don’t have enough room on your slab as specified. You could build a bigger slab, but it would need to be in proportion to your base, which would also need to be built to a wider dimension to keep it under the walls of your oven and bear the load properly. A secondary consideration is that you would need about 70 more firebrick and corresponding amounts of the refractory mortar to build a second arch. Add another 25 firebrick for the third arch (for the door). The refractory tiles aren’t a bad idea, though I see the designer has them placed at right angles which could lead to catching the edge of a peel if any of them heave.
I believe the layer labeled “vermiculite render” is actually similar to the Vermiculite/Portland cement mix specified for the insulation layer in the hearth slab. First, I’d substitute Perlite for Vermiculite to avoid an insanely-long curing time, and I would mix it with Type S mortar rather than Portland cement. It would add the benefit of extra insulation, because in this design there isn’t going to be enough room to do a full double-layered mineral fiber blanket. The disadvantage is that the Perlite or Vermiculite will weaken the scratch layer, which is the foundation of your protective shell over the oven. With a single, thin finish layer the stucco layer would be vulnerable to impact or crush damage.
The chimney should not move closer to the outside arch. Your fires will be toward the back and along the sides. Fresh air comes in through the door opening or around the edges of the door and travels along the floor. Hot exhaust gases travel up and along the apex of the arch, then forward because of the pressure from the fresh air below. The chimney opening is placed to intercept those hot gases before they reach the door. Placing it any closer means a 50/50 chance that they will simply get through the door rather than being drawn up.
The big advantage the BrickWood design brings to the table is its simplicity. I don’t see the double arch adding much to the design, and the construction looks much more elaborate than it needs to be.
Hope this helps, Ana. It’s great to look at other designs!