You should be able to do what you want to do with only 2 uv sets: - Your first set you're free to use as you wish (tileable details for example). - Your second uv set has to be "lightmap compatible". That means no overlapping polygons and all within the 0..1 range. Then remember to uncheck "Generate Lightmap UVs" when…
There also something to note about lightmap seams, as it is something I got hung up on for the longest time. Until you get your object fully textured, it's best not to worry about the glaring lightmap seams you might get. So matter what object you make, the seams will always look bad with a solid color. When I was first…