A prewar twin 6-inch cruiser turret was possible but unnecessary

Could Vickers and the Royal Navy have fielded a workable twin 6-inch cruiser turret before the First World War?

Yes, but there was no pressing operational need to accept the development and production disruption.

Vickers and the Royal Navy had enough technical experience to make a twin 6-inch mounting feasible before the First World War. The obstacle was not an unsolved engineering barrier. Existing cruiser arrangements met the service's needs well enough that no urgent demand justified the change. Had it been adopted, the already strong British cruiser force would have gained still more concentrated firepower.