Hello,
As of 4.11 there is a material function called “Derive Tangent Basis” that you can use to derive a tangent basis based on a specified UV set. Here is some more info copied from an internal email that will hopefully make it over to docs soon:
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A limitation in UE4 is that normal maps have to use UV0 or they will be incorrect because the engine only stores one set of tangent vectors from UV0. This is a material function that can derive new tangent vectors using the specified UVs so that you can use a normal map with any UV channel.
Note that for this node to work, your material must be set to “Tangent Space Normal = FALSE” since the function returns a World Space Normal. If you need to combine this normal with another normal that is using UV0, the UV0 normal needs to be transformed from Tangent to World."