The normals of the UVs will be inverted. The mesh normals usually won't. If normal and bump maps are displayed correctly depends on the renderer or real time engine used.
Never used Unreal, but I don't see why it shouldn't be able to deal with mirrored UVs. Are you having any problems?
I am still a little confused. If I understand correctly, you can't "flip and stack" because the tangents would be reversed on the flipped UV's: concave normal details would become convex and vice-versa (regardless of engine)?
-So the answer is 'no'?
EDIT: I'll be using Painter to create the normal map.
I think it is still a problem with only certain pieces of software not exporting the tangents and binormals correctly. I don't bake in 3dsmax any more, I use substance and I haven't run into in a while, but then again I don't mirror UV's as much as I used to. Personally I wouldn't use 3dsmax to bake anything, RTT is broken and bugged so bad it's unusable, plus it doesn't output half of the maps that I actually need.
"concave normal details would become convex and vice-versa (regardless of engine)? "
In theory, yes. But since that's easily fixed by identifying the flipped UVs and inverting the normals in engine that usually isn't a problem anymore. To be safe, you should just bake the unflipped, unstacked normals though, so move the other UVs one tile over or delete them while baking. And using an aligned renderer and engine like Mark suggested is definitely a good idea.
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In theory, yes. But since that's easily fixed by identifying the flipped UVs and inverting the normals in engine that usually isn't a problem anymore. To be safe, you should just bake the unflipped, unstacked normals though, so move the other UVs one tile over or delete them while baking. And using an aligned renderer and engine like Mark suggested is definitely a good idea.