With custom coding, you could create CSS media queries that show 2 pages side-by-side at a certain window size.
This is probably more advanced than I can cover in a short answer.
Now automatic with the Flipbook format.
I've got part of the answer. I assume you're interested in a horizontal slider, as am I. And I want back and next arrows.
The setup: I have a page width of 512px giving a double page spread of 1024px.
Currently the width of the page container .anythingSlider is determined by inline CSS:
<div class="anythingSlider anythingSlider-in5 activeSlider" style="width: 512px; height: 768px;">
Widening this to 1024px display's two pages side-by-side. This can be done with CSS:
width: 1024px !important;
... and use 512px for smaller device widths to get back to a single page display.
That was the easy part. The part I'm stuck on is the page navigation. The back/next arrows only advance 1 page at a time, but on desktop and tablets in landscape we want it to advance 2 pages at a time. This requires some device detection (orientation or device width) and changing the behaviour of the anythingSlider back/next arrows. Both are beyond me.
I had another inelegant solution using two page navigation buttons per page: one with the next page and a hyperlink to the page after that (using this solution for linking to a specific page). Then use CSS media queries to hide the button not needed based on device width. It works but there is no page transition on the hyperlink.