But won't you have to duplicate all the morphs as well?

You should not need to change your strategy if the only problem is that the order (topmost) of the doubled polys is ambiguous.

First there is the case where the second skin isn't being used. Well in that case, it should be 100% transparent, so who cares which is on top?

Second case is where the second skin is used, but not displaced. In this case there is ambiguity, but in this situation there is no point in having the second skin. Whatever combined materials are desired can be implemented with one polygon layer, using a blended shader. This has been demonstrated already - foam, oil, whatever. Second skin doesn't help with these so don't bother.

The third, and only legitimate, case for the second skin is if it will be displaced in the shader. In that case, the layering will not be ambiguous at all.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)