Double Crossbow is a technology in Age of Empires II HD: Rise of the Rajas that is unique to the Khmer and can be researched at the Castle once the Imperial Age is reached. Once researched, it makes Ballista Elephant and Scorpions fire two projectiles instead of just one.
It further bolsters the strength of Ballista Elephants and the Khmer's already impeccable Scorpions (Team bonus +1 range, access to Siege Engineers) against mass Infantry Units.
To offset the power of this technology a little bit, the second projectile has a slight cone spread and deals a little less damage than the first one.