A double crossbow is an unusual contraption that consists of two individual crossbow mechanisms built one atop the other. You can fire a double crossbow twice before needing to reload, potentially allowing you to make multiple attacks if you can make more than one attack in a round.
You draw a heavy double crossbow back by turning a small winch, which draws both strings simultaneously. Loading a heavy double crossbow is a full-round action that provokes attacks of opportunity.
Normally, operating a heavy double crossbow requires two hands. However, you can shoot, but not load, a heavy double crossbow with one hand at a -6 penalty on attack rolls. You can shoot a heavy double crossbow with each hand, but you take a penalty on attack rolls as if attacking with two one-handed weapons. This penalty is cumulative with the penalty for one-handed firing.