Surely a double barrel shotgun would fire twice or else it would be a triple barrel shotgun...
What about reload times?
Actually, the way the DS is now is pretty good, even in reality. Two shells at short range will always deal more damage than one because of the SHAPED CHARGE (google it!) effect of shotguns at short range, where raw damage rises at an exponential equation to power.
DS Ballistics shots reveal two interposed elipses instead of a circle of just one shell.
The interposed area (like a cat´s eye) reveals a higher pellet density, higher force applied to the same area than individual shells.