I really don't think there's a built-in way to do it, but I challenged myself to find the shortest workaround and came up with this:
- Click at the very end of the document and choose Insert > Page Break, making a blank second page*.
- Click File > Print.
- In the bottom-left corner, click PDF > Open in Preview.
- Note the thumbnails in the sidebar. Hold Option (Alt) while dragging the first page's thumbnail to duplicate it below itself.
- Click the blank page's thumbnail (now the third page) and press Backspace or choose Edit > Delete.
Now you can print using the two-pages-per-sheet setting you found before:
- Click File > Print.
- Click Show Details (if that's not already done).
- Click the dropdown that says Preview and choose Layout.
- Click the Pages per Sheet dropdown and choose 2.
I'll leave it to you to judge whether that's an improvement on copying and pasting the document within Pages!
*Unless the document is at least two pages to begin with, you can't Option-drag thumbnails to duplicate pages in the Preview sidebar.