bucket seats in a 64 Valiant and master cylinder questions

Thanks Searcher. I looked at the article that 64conv65hard also sent me. Kind of confused. There are not enough holes in that prop valve for another brake line. Im guessing here that the two are joined off the MC and are going into the prop valve in a single hole?? If so how does that differ from just using the single bowl MC? Or am I looking at this all wrong lol??