Not sure what you are trying to do there.
If you want to adjust the Opacity of the outer shell.
First. Figure out which UV is right - preview the output of the UV - you can do this by adding an add node and adding 0 and clicking preview from the rightclick sub-menu of the node.
What you want to see is the inner shell being black. The outside being white.
From that - you apply the result to whatever you want to do as a UV.
If you want to add transparency to the outher shell, make sure the material domain is correct for what you want.
Then lerp just like you are doing against something thats not 0 to see results.
Whatever areas are white in the preview are whatever the lerp will result into.
The reason for having 2 UVs is that you need to be able to map areas to pixels differently for the outer and inner shell while also being able to isolate them.
So, one UV will have the points of the outer shell in a 0 to .01 range, while the other will have the same points in a .01 to 1 range.
To apply a texture to it properly, you want the uv that is in a .01 to 1 range.
Or else, the same pixel would be applied to the whole mesh part, making it a unique color.
Maybe that is what was happening or why you are confused about it.