Imagine 10 engines of equal size. You could say after one or two were running you could never go any faster and the extra engines would have no effect. On flat water it would be true. As the sea state increases it takes more power to maintain the same speed through the water and the added engines will help maintain speed. More props in the water will increase power. Adding engines would let you increase pitch
on flat water but not also in an increasing sea state. The problem is the sea state is a larger variable than the pitch of the prop and an automatic prop can't really account for the whole dynamic sea state change unless you could dial in the pitch as a function of sea state. That would make the propulsion
system pretty complex for a sail boat but not for a large power boat
because they actually can do that. It's not an automatic function of the prop though. The whole sea state effect computation is a bit beyond high school
physics. Even my autopilot
can't compute it automatically. You have to set it.
You need a balance and you can clearly argue what the balance should be but you would not maximize speed to a flat water sea state in the design would you? You would clearly lose efficiency in an increasing sea state so I would expect to lose a little bit in flat water to have more when it really mattered. Over pitching the prop has some serious negative impact on transmission
and engine performance / longevity. Under pitching causes cavitation to lose efficiency too. Taking everything into account it's probably true that the original boat design was correct.
Going back to the original question, one prop won't do more work
when two are engaged. It's not like the props "know" about each other. They would always both perform the same amount of work based on the throttle and each exhibit the same inefficiency. The order that you start them is meaningless. You would have added power you can not realize in terms of added speed, but it would always be there though wasted from your point of view. As a lone wave strikes the bow in flat water the time to regain the prior speed would be quicker with two props. That would be how you sitting on deck
would know the difference.