Answer: Limbs typically do not just fall off fruit trees. If limbs die, they may be attacked by insects or diseases; if they are missing, perhaps deer or other munching creatures are removing them for you. I would prune each tree to a symmetrical form and wait for new growth to appear. They may look more like shrubs than trees without a central leader, but at least you'll get additional branching and, hopefully, fruit. Best wishes with your trees!
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