Ah right, I’ve opened up the project now - they’re all staying on separate pages because there isn’t really space for them to go onto 2 pages. If you reduce the staff size, or increase the page size, that might happen automatically (as long as you’ve got the setting to allow flows on previous pages).
I noticed that all your pages have overrides in Engrave mode - it looks like you added the title and deleted flow headings directly on the page. Although that’s the most obvious way perhaps, it’s not the best way as if the 3rd page has overrides and you make the music fit on 2 pages, the 3rd page will remain there, empty. Here’s some information about page overrides.
Instead, you can enter the project title information in the Project Info dialog (the information on the first page will get updated automatically), and hide/show flow headings (in this case, you’d want to hide them!). If you don’t want to see the little flow heading at the very top of the page (for pages 2 onwards), you can edit the Default master page and remove that token. You’ll get the best results out of Dorico when you try to do the majority of your edits at a global/default level - that means you have to manually edit fewer individual pages.