Unless you are shooting from the ground, one lung will probably be a high hit and the other a relatively low hit. That being said, there are two factors that compromise the lungs - one being blood in the bronchii and alveoli (small terminal air sacs where respiration occurs) and the other being the puncture of the lung itself which causes air to leak outside of the lung into the chest cavity, causing tension on the adjacent lung and lung collapse. I would suspect that the latter is more important in causing the deer to bed down/die immediately, however I don't know if anyone has ever studied this.