When zoom level is set to Fit Page then two pages in side-by-side mode share the same ratio. If two single pages are not of exactly the same resolutiom, it looks somewhat disruptive and defeats purpose of side-by-side mode (read like book).
Please would it be possible that both pages are in fit-to-page zoom?
I understand that this can go wrong if sum of both pages is wider than screen, but from my experience on nowadays wide monitors this "wider" scenario does not really ever happen... Possible solution would be:
1. compute Fit-to-page zoom as is done now
2. take the vertically smaller page and zoom it to maximal possible height(<=screen height) so that sum of both pages are still not wider than screen.