Why twin gun mounts spread slowly to smaller warships

Why were navies slow to adopt twin mounts on destroyers and light cruisers, and when could small twin mounts be implemented successfully?

Two guns multiply hoists, ammunition feeds, crew and magazine demands; workable small-ship installations emerge around the mid-1920s once the added complexity becomes worthwhile.

The panel follows the consequences of doubling a mounting through hoists, crews, ammunition supply, magazines, armor, weight, and internal volume. On a destroyer or light cruiser, designers had to decide whether that complexity delivered more value than another simple single mount elsewhere. A closing answer places successful small twin-gun implementation around the mid-1920s, when the supporting technology and ship designs could absorb it.