Being able to dual wield heavy weapons doesn't make the Orks better at ranged combat. They still have poor ballistic skill and in situations like tight corridors with not much open space they find themselves in close combat with limited ability to fire heavy weapons soon enough.

Being able to fire twice increases the odds of you hitting by a rather large amount, and the fact that there exists situations where it doesn't grant you a massive advantage doesn't mean that it's not incredibly good. All it takes is a displacer field to set yourself up for an endless stream of point blank shots against close combat enemies.