I dunno if you guys are still having this problem, but what worked for me was moving the mirrored UV's over into 0-1 space. I bake with my mirrored UV's in the opposite quadrant to avoid errors and noticed the seam when i imported with those UV's in separate quadrants. Moving all UV's into 0-1 space fixed it for me.
That is not true when it comes to unreal 4 (or most game engines right now). The unreal mikkspace workflow expects to use mesh normals from the file and calculate tangent using mikkspace. FWIW, source engine, unity, and unreal 4 all expect to use mesh normals from the low poly model and then calculate tangents. This thread…