Well I already googled it. And this is how. I had an action character named Cajj (Cage) for a short comic I used to do called Gold Team. It took place in the year 5006 AD and Cajj was the leader of the U.S.'s military (yes the whole thing). He used double pump shotguns. Here's how he used them:
1He would hold one in each hand (obviously)
2When he shot, to pump them, he would throw them slightly in the air and flip them so that he caught them by the pumps.
3There he would just force the shotguns downward leaving the pumps in place (thus pumping the shotgun) (if you have ever seen the movie Evolution, after the guys at the mall shoot down the dragon alien the hold their shotguns by the pump and pump them. Cajj pumped them like that only with one in each hand
4 He would flip them again to be able to fire them
So there you have it. Thats why I wanted a double shotgun model and there's how ou would hold dual shotguns. The whole pumping phase in real life would take just as much time as a normal pump so don't think that it was a slow process.