Enables you to insert and fire two bolts of your choice, if you have this mod attached to your crossbow. As usual, you can find the mod (rare drop), as well as find the schematic to craft it. Not obtainable by a trader or a skill. The recipe itself is expensive, because it will make you perform double damage – and even lets you fire two exploding or flaming arrows too.
P.S.: There is a mod, simply extending the magazine size. This was too OP for Claymore, so he decided to do his own variant.
The forum topic of the mod is here.