Usually only one of the two ships served with the fleet while the other trained, experimented with minesweeping, or sat in reserve. Early-twentieth-century mobilization could give defenders warning because a major assault force required a conspicuous concentration of ships and personnel. A defender could call up its own reserves while the attacker assembled, turning visible mobilization into preparation time. That made reserve activation more plausible than modern assumptions about instantaneous attack can suggest.
Notable Point · 5:00-7:02
Why one minelayer could remain in reserve
The alternating active-and-reserve arrangement made sense when an attacking fleet could not mobilize invisibly.