Trouble with light mapping on duplicate objects

I'm trying to light map the base of a snow globe with tree shadows on it. The number of trees gets bigger as the game goes on so I have the trees on separate layers (tree group 1 is on tree1 layer, tree group 2 is on tree 2 layer) I'm trying to light map each copy of the snow globe base with each grouping of trees, but it just ends up baking the same light map for both snow globe bases. Here's a picture of what I'm trying to do and the issue (by hidden I mean deactivated):

http://i.imgur.com/mehAcXR.png

Anyone have any suggestions? Thanks!