Well, it's all in AUM 56, so, in the entity, skin 1 is the base skin for fallback support, 2 is the mask.

I define the 3 skins to be multitextured by script. The one masked by green is the one to be normalmapped. Basically, its tex 4.

It will be much easier if you download AUM 56, which includes the multi-texturing code I am using.