Why twin naval gun mounts matter

Why favour twin gun mounts over single mounts on modern warships?

Twin mounts can deliver more rounds during the brief period when volume of fire is most likely to decide the engagement.

He distinguishes a naval gun from a land system by the ship's ability to carry heavier mountings. In the kind of short, intense engagement he expects, putting more rounds in the air quickly matters more than conserving the same ammunition for a longer firing period. That leads him to prefer twin 40 mm, 57 mm, 76 mm, or 4.5-inch mountings over the single mounts common on current British escorts.