... why no range shells? I'm pretty sure hitting someone with an unfired pellet shell does a lot less than firing it, and if your specials can go off that easily it's probably a bad idea to be holding it when you hit someone.
Other than that, multipower, 1 charge, some huge number of clips...
... and 5 points to double the number of barrels. (and clips)
I'd give it a -0 limitation, must preselect loaded clip (although it can be different for each barrel)
and a +0 advantage, can swap clips between the two barrels.
Of course, since it'd take a partial action to reload for each barrel, you might want 2 charges with clips instead... that way it'd only take a single action to reload, but I'd say both barrels would have to have the same type of shell then. Your GM might rule otherwise. Although then you'd also need to add autofire instead of being able to fire both single barrels as a multi-power attack...