Assuming you give both barrels, there's no attack roll so...
yeah...
Alright I'll take a stab at it:
Scatter Weapon Quality: A weapon with the scatter weapon quality can shoot two different types of ammunition. It can fire normal bullets that target one creature, or it can make a scattering shot, attacking all creatures within a cone. Cannons with the scatter weapon quality only fire grapeshot, unless their descriptions state otherwise. When a scatter weapon attacks all creatures within a cone, it makes a separate attack roll against each creature within the cone. Each attack roll takes a –2 penalty, and its attack damage cannot be modified by precision damage or damage-increasing feats such as Vital Strike. Effects that grant concealment, such as fog or smoke, or the blur, invisibility, or mirror image spells, do not foil a scatter attack. If any of the attack rolls threaten a critical, confirm the critical for that attack roll alone. A firearm that makes a scatter shot misfires only if all of the attack rolls made misfire. If a scatter weapon explodes on a misfire, it deals triple its damage to all creatures within the misfire radius.
This twin-barreled shotgun can be shot either one barrel at a time, or both together as one attack. A double shot that fires bullets is inaccurate, and takes a –4 penalty on both attacks. A double shot that fires bullets targets only a single creature and increases the damage of each barrel to 2d6 points (Small) or 2d8 points (Medium) for a total of 4d6 or 4d8 points.
A double-barreled shotgun uses metal cartridges (loaded with either a bullet or pellets) as ammunition.
So I'm going to go with: deal damage like it was bullets to each creature in the blast, since that's what scatter damage is typically.
However I'll readily admit that I'm not 100% on that.
So automatic 2d8, no saving throw?
automatic 4d8 (2d8 per barrel) no saving throw (since there never is one with scatter effects barring dragon's breath rounds).
However you also won't get bonus damage from: vital strike, sneak attack, weapon specialization, point blank shot, deadly aim and so on. Class features fortunately (in general excluding sneak attack or other class features that are precision based) will still increase damage.