What makes a twin 5-inch mount time-consuming

How difficult would it be to develop a twin mount for a modern 5-inch naval gun?

It is feasible but time-consuming because the paired mechanisms, barrel arrangement, and maintenance access must all be designed together.

A twin mounting does not require an unprecedented technology, but it does demand careful arrangement of two guns and their handling systems. Designers must choose whether the barrels move independently, mirror mechanisms to use turret space well, and leave enough access for maintainers; that work is complicated and slow even when it is achievable.