The material below is Open Game Content from a 3PP source
Double Crossbow: This heavy weapon fires a pair of iron-tipped bolts with a single squeeze of the trigger. Due to its size and weight, you take a –4 penalty on your attack roll if you're proficient with it, or –8 if you're not. If the attack hits, the target takes damage from both bolts. Critical hits, sneak attack damage, and other precision-based damage only apply to the first bolt. Reloading one bolt is a standard action; the Rapid Reload feat reduces this to a move action. Crossbow Mastery allows you to reload both bolts as a move action
|Exotic Ranged Weapons||Cost||Dmg (S)||Dmg (M)||Critical||Range||Weight1||Type2||Special|
|Double crossbow||300 gp||1d6||1d8||19–20/×2||80 ft.||18 lbs.||P||—|
1 Weight figures are for Medium weapons. A Small weapon weighs half as much, and a Large weapon weighs twice as much.
2 A weapon with two types is both types if the entry specifies “and,” or either type (wielder's choice) if the entry specifies “or.”