Having a good, clean 2nd UV set is ideal for setting up a high quality lightmap inside of UDK.
I have created a video posted over at 3dmotive to help you with the process of copying your first UV set into your 2nd UV set, as well as some "do's and don'ts". You can find this tutorial here:
Hourences has also saved me the trouble of completely re-writing this tutorial. This will guide you through additional steps and the process behind setting up your 2nd UV channel and lightmaps inside of Unreal. (FYI, there is no reason to apply a material to that second UV channel when it comes to lightmaps...)
You can find his written tutorial here: