If the verts are merged/welded between the halves, you could try not merging the mirrored half (this requires re-baking the unmerged variant), ie. the edges along the mirror seam are open.
It's also possible that a render engine doesn't support mirrored normal maps. If you still get the inside-out effect on the mirrored half, even if it isn't merged, then that's probably the case.
You might still get a visible seam, depending on how the polys are placed, but knowing whether the mirrored half is inside out or not will answer the question if can handle mirrored normal maps or not.