To solve light bleeding use 4 pixels between each shell not 2. To solve seams between modular meshes: Combine meshes in Maya and import into UDK as one larger mesh. Also merge the verts along the seam, and merge the 2nd channel lightmap UVs so that they are one continuous shell, then use a larger lightmap size if needed.
I mean sewing the UVs in the 2nd channel (the one for lightmaps) together so that there aren't seams. Looking at the OP's example: Instead of having 2 instanced meshes side by side you'd want to have one mesh with seamless UVs where the lighting seam the middle is. So in this case you would want to have 6 UV shells total…