the enclosure technically is it's own structural system, to a degree. But, as has been pointed out in the Larsen Truss thread, we also rely on our enclosures a lot of times to attain the shear bracing needed to appease local building codes, which is difficult to attain in a tradition timber frame which is designed to flex.

With such in mind, I agree with bmike that the intermediate layer, the one closest to the TF, should be able to provide this bracing, and the outer layer can be whatever is handy or suited to the job.

Out of curiousity, why do you propose the inner studs be horizontal?

Conceivably, could doing so provide adequate lateral bracing, and thus eliminate the need for structural sheathing to resist shear? If so, this is a great idea, as it would eliminate the need for any costly sheathing, and the intermediate layer could be replaced with foam board.

I tend to use a lot of long, complex sentences, in a way which is not proper for English grammar. o well.