I believe that you are not supposed to be able to use upgrades on your light armor while in power armor, after all your jet pack on your light armor allowing you to fly while in your colossal war-mech is just plain silly. But, as far as I know this hasn't actually been addressed in the rules. However, I would definitely rule that all upgrades on light armor are non-functional in power armor and, for that matter, upgrades on an Android (again jet packs), etc.
Additionally, let's say you can wear both force fields, then we can take a page from classic D&D and Pathfinder, you could have both but they don't stack, after all in Pathfinder two false life spells wouldn't stack. It's pretty clear that multiple force fields is not intended by the game design.