When Dealing with lightmaps and UDK. If one is not careful you can get what appears to be shadow bleeding. This can turn a good looking mesh into one hell of a ugly one if not careful. One of the first mistakes that newer artist do when creating their second UV channel for baking lightmaps is just going to flatten mapping. Bellow shows the wrong way to do this. Just hitting flatten mapping causes the UV's to become fragmented. Every uv island is going to have some sort of lighting seam around it. When planning for lightmap uv's it is best to have as much of it connected as possible.
A few key things to remember.
1. When doing a lighmap unwrap, you CAN NOT have any overlapping UV's.
2. Be carefully of the spacing between UV islands.
If you keep these things in mind and take some time to do a proper UV for the second channel. You will have much better looking models within UDK.