Villeneuve knew from the very beginning he wanted to turn Dune into a two-part film, but he hadn't quite figured out how to. In a new cover feature with EW, Villeneuve explained why the task was actually rather challenging when it actually was time to do it. It all came down to ensuring the first part could stand on its own without completely severing itself from what comes after. Villeneuve said:
"The book is so dense. It's a story that takes its strength from details. So one of the very first things I said to Legendary was we should not make one movie, we should make two. The question was, when do we stop the first part? That took a while, sculpting the end to make sure the audience will feel like they've been through a full journey, a full arc for Paul Atreides, but at the same time they would feel there's something else coming after."
]Naturally, Herbert didn't write his first Dune book with the intention of ever splitting it into smaller segments. Paul's arc stretches across the entire novel, so it's easy to see how Villeneuve might've struggled with finding an acceptable midpoint. Fans have been trying to guess just where the movie will cut off, but they've only been provided a handful of clues. For example, Zendaya's Chani has a very small part in the first Dune, but should the sequel get greenlit, she'll become the protagonist. That might be a big indicator of when Dune will end.
Uhhh...it was originally published in two parts. That information is not difficult to discover. It is mentioned in the first line of the Wikipedia article. It was two magazine serials, and the end of the first one and the beginning of the second one were seperated by 10 months.