Just to confirm we're talking about the same thing - I'm not talking about the iOS feature called "Split View" where you can have two apps side-by-side, or in iPadOS two windows from the same app.
What I'm interested in is if I open just the Books app and then open a PDF within it, can I choose to view the PDF either one page at a time, or two pages (a spread) at a time?
Thanks for taking the time to answer!