Slugs and double barreled guns don't often go together well. It's all about something called "regulation". No, not the law. Regulation is the way the barrels are joined together so that the point of impact from both barrels meets at a common place. Barrels that are properly regulated (i.e. normally high dollars) have the point of aim and point of impact of both barrels join at somewhere between 30 and 40 yds. Less properly regulated barrels might never have a common point of aim and impact. Their shot strings could actually get farther apart as you get further away from the barrel.
With shot, having a regulation problem isn't always a big deal. However, with slugs, poor regulation has a huge difference in where that slug actually goes downrange. The two barrels may shoot slugs to really different points of impact.