12 inches is not a firm maximum wall thickness. You can go thicker - particularly in drier climates. A common rule of thumb is that the walls dry about one inch per week (6 weeks for a 12" wall). However, I think our 12" walls dried in 3-4 weeks. If you have decent drying conditions you could go thicker than 12" if you wanted.
Another thing is that you can change the mix to get more insulation value or more mass. Typically, people make a lighter mix for the north walls and heavier for the sunny sides. Also, the light clay straw walls perform better than the R-value would indicate due to the effect of the thermal mass in the walls.
I would recommend against layering the walls with dry straw in the middle. First, it would be a pain to install with any consistency. Second you would be making planes of weakness parallel to the wall surface. You could easily get sections that spall off. One of the beauties of light clay straw is that the material makes cohesive, uniform density walls. Layering the walls would disrupt the cohesiveness of the walls. You are better off making the walls thicker or using a lighter mix.