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I see, well, how about letting the user choose, and act according to it? a .X will never tell you about the second texture, afaik. Would be any troublesome anything like that?
"There is nothing truly useless, it always serves as a bad example". Arthur A. Schmitt
Anything like what? Yes, if you mean like you proposed. Because it would mean to invent a whole new material system and file format. Only a little if you mean my suggestion. Though it will require manually tweaked .x meshes.