Umm, sorry. I don't think Duoderm IS even gas permeable, though I know tegaderm is. Actually I have found evidence it is not semipermeable from australianprescriber and emedicine: Medscape: Medscape...
Sorry this is so late gettingupthere, but I think you were right the first time. I think layering them would still reduce the gas permeability. Both Tegaderm and Duoderm are intended to be the only...