HappyDaze said:

Twin-linked sounds more appropriate.

I too think twin-linked would be better. The precision dual firing that Storm represents is an almost lost art in the Imperium.

I would use the storm rules for all weapons with two closely mounted barrels firing simultaneously. Either both shots hit, or they both miss.
I can't see why it should be +20 easier to hit with a double barreled shotgun and harder to dodge two hits.