It may not be all that difficult. Select both trees and go into solo mode now select the one tree that you want un-AA and do a render mask. Only those parts that can be seen will be white. It's what I did because the Aslan (in Refueling on Planet X) was in front of the tentacles. I took them both into solo mode then unselected the Aslan and rendered the mask, where his legs were in front of the tentacles showed as black. One note here, to do a object mask with metaballs that are grouped. You can't select the groups, the mask won't turn out right, you have to select just the metaballs.