I've a passage from one of my other favourite brewing manuals, "The Principles and Practice of Brewing" from 1907. I love terchnical brewing books from this period. Mostly becauise I can still just about follow everything in them. Later 20th century works lose me when they go organic chemistry crazy.
When the wort is boiled at twice or three times, or when it is distributed over several coppers, each copper should have its due proportion of hops, calculated on the quantity and respective gravity of the wort of each.
"The Principles and Practice of Brewing", by Walter J. Sykes and Arthur R. Ling, 1907, pages 522-523.
So Sykes and Ling are in the two additions camp. The first 15 minutes after the start of the boil, the second 20 to 30 minutes before the end. Have you noticed something? No two authors have come up with the same recommendation yet.
Note how, rather than reusing the hops from the first copper in the second, they suggest dividing up the hops based on the gravity of the worts. This is the scheme that Barclay Perkins followed, putting rather more hops in the first copper, where the highest-gravity wort was boiled.
Those two patented devices for hopping beer during fermentation sound weird. I wonder if this was a replacement for or supplement to hopping in the copper? I'm pretty sure neither piece of equipment caught on.