That's interesting, rjjack. I may have to experiment with that approach. Which way do the "inside" polygons face? It's been my experience that the cloth simulation doesn't deal so well with collision from the way the polygons face - it's as if it's been biased to deal better with the inside of clothing, which is reasonable since that's the usual situation. I imagine the two layers would tend to collapse together during simulation, which may be why it doesn't give as much thickness as you'd expected.

TG, it's genuinely double sided polygons that the cloth room really doesn't like. I can see that this would simulate all right if one was careful, since it's really two dresses layered together. Displacement doesn't give you thickness at the cloth edge the way this does.