UV layouts are not skin related but mesh related. Each vertex has two sets of UV coords available. You will need to set both UV set in the model, you know.
Code:
in float2 inTex1 : TEXCOORD0
in float2 inTex2 : TEXCOORD1


Those texture atlas examples are just examples. I did not think to much about them.

In the first example, you will probably prefer to mantain the model UV coords filling the texture, as they usually are, and simply scale them for your needs.
Code:
float2 diffCoor = float2 ( inTex.x * 0.5f, inTex.y );
float2 normalCoor = float2 ( diffCoor.x + 0.5f, inTex.y );


I was just trying to explain that you can use the models UV coords for your advantage but it does not needed to be that way. Blank spots should not be in the planning, true wink