I've posted this elsewhere, and I'll repost it here:
I know what Villeneuve has said in the pass concerning back-to-back shooting of the two movies, but I'm not convinced. I still believe he is shooting both movies simultaneously. Here's why:
1 - The very long shooting schedule: this one began shooting more that 1 year and half before release. Blade Runner 2049, for example, with the same director e and similar crew, started shooting in July and was released in October in the following year, and was surely also an extremely complex shoot. Dune has a 6 months lead compared to Blade Runner between start of the shoot and release date. This suggests a longer shooting period and therefore the possibility of two movies
2 - The Cast: characters that have nearly insignificant roles in the first part of the novel, like Stilgar and Chani, have been cast with actors with quite a considerable public profile. I doubt the casting of the two characters would be announced if they didn't feature considerably in the shoot.
3 - Cost effective - with so many locations, settings and characters shared between the two halves of the book, a back-to-back shoot would make much more financial sense, I think. Otherwise, sets like the Arrakeen Palace would have to be left standing for a prolonged period of time awaiting the start of the shoot of the second part.
4 - If the casting of Irulan or Shaddam IV is announced in the next few weeks, I think that will give us a very strong indication that the two parts are indeed being shot back-to-back