Unfortunately, our printing partners don't support spread layout out of the box and that's why we don't offer a two-page spread design option. However, there's a workaround you can use to create spread illustrations.
To illustrate your books in spreads, you'll need to split your spread illustrations into two halves and upload them to facing pages. This way you'll create a two-page spread illustration. You can then verify the placement and adjust the layout by switching to Spread Layout view mode. To do that, zoom out to 50%.