I’ve ran into this issue not so long ago. Mirrored UVs are absolutely fine in UE4, however, there are few caveats, including limitations of the software you are exporting from.
Firstly, you have to ensure, that your modelling software is exporting mirrored tangents and binormals correctly. Last time I tried, blender did not by default.
Then You have to ensure that UE4 importer is set to import tangents and binormals.
You should get a seamless mirrored mesh.
If with this settings, you are getting the result, like the one you have on your picture(one side has flipped normals) It most likely means that your modelling software and/or its exporter is not exporting mirrored binormals correctly.
In this case, you can set UE4 importer to “Import Normals” and it will automatically generate binormals properly, however the drawback is that tangent space will be slightly discontinuous along UV edge, and you will notice a slight seam in normal map.
Do note, that you do not need to set sampler tiling mode to mirror.
I am not sure which packages and their versions are exactly capable of exporting mirrored tangents/binormals properly, but I can safely say that not all of them are. In fact I would be really grateful, if anyone would put up a list with that.
As Luos mentioned, these complications are main cause for mirrored UVs not being popular, when used with normal mapping.