This is an issue of the lightmaps not lining up with eachother, which can be (usually) corrected by making sure the 2 displacements in question first have their vertices aligned, then aligning the textures. If the vertices are not aligned (IE: you have 1 displacement that is much wider than the other) the lightmaps won't line up and this is the result.
Alternatively, playing with the lightmap scale itself will sometimes mask that effect.