My understanding is no overproof on airplanes. The amount you bring back is based on liters not bottles but I have never had anyone look at the volume of the bottles. Two bottles each won't be questioned. The real fun comes when you need to quickly move them to the checked bags and packing them well enough that you don't have one break. I always pack a folded beach towel right in the middle of the checked bags to slip the carry-on boxes into.