'Derive tangent basis' and 'Blend Angle Corrected Normals'

I’m attempting to combine normals from multiple UV channels and can’t get it to work correctly. Once I blend the normals from two different UVs lighting gets messed up. Here’s what I’ve done so far:

  • I have a cube with two sets of UVs on channels 0 and 1.

  • ‘Tanget space normal’ is unchecked on the material

  • Testing just UV0 with transform tangent-to-world looks as expected:

  • Testing just UV1 with derive tangent basis also looks as expected. There’s supposed to be a really faint cross thingy on each face of the cube:

  • Once I blend the two, things get mighty strange:

Any help is much appreciated!