The Pathfinder core rulebook has this to say about Light Crossbows:
"Normally, operating a light crossbow requires two hands. However, you can shoot, but not load, a light crossbow with one hand at a –2 penalty on attack rolls. You can shoot a light crossbow with each hand, but you take a penalty on attack rolls as if attacking with two light weapons. This penalty is cumulative with the penalty for one-handed firing."
This is particularly relevant to Shadowshooting weapons, since one can actually dual-wield Light Crossbows in that circumstance.