How to deal with spreads

[kaworu1986]

I'm writing a tool to generate fixed layout epub3 files out of collections of images, to simplify and automate creation of graphic novel/manga epubs from raw assets.

One issue I am coming up against is spreads: it is pretty common them to be generated as a single image having twice the usual width as opposed to separate ones for each page. Is there anything in the epub3 fixed layout specification that accommodates this use case?

Adding "rendition: page-spread-center" as attribute to the affected spine item is the closest I could find but:

1. By spec, it should only tell the reader to display the page alone (if it usually puts 2+ pages in the same screen) but not to make it twice as large.
2. It doesn't seem to work anyway: both Edge and iBook on macOS and iOS just ignore the attribute.

This is on an epub that passes validation