This is currently only possible in our download software:

Select a photo placeholder on the page whose page layout you want to save and click on the "Save Layout" function in the Arrange toolbox in the "Layout Special Functions" area. Now this custom page layout will be saved directly into the main category Page Layouts. Additionally it gets a green star. This way you can recognize your own page layouts faster. You can also find them in the page layout category "My layouts".

You can then use your own page layouts for other WhiteWall Bildband projects. However, you can only save your own page layouts for a single page in the image book, not as a double page.