It's not two bullets at the same time. It's a gast gun. The weapon combines two barrels into a single mechanism so that the recoil from one barrel charges and loads the next. It's automatic fire but the sequence is 1,2,1,2,1,2 accordingly. It can effectively double the rate of fire, from 600-700 for a typical machine gun to 1200-1300 while reducing barrel wear by only firing one every two rounds. The Soviets resurrected the gast principle in the 50's onward to derive an autocannons with high rates of fire but excellent reliability. The US went the other route and utilized the Gatling weapon.