[itsme]

It's not complicated, but needs some manual actions.
PDFBookmarks contain just the starting page:
Song 1 - 1
Song 2 - 1
Song 3 - 2

A minimal CSV file contains the title and the page range:
title;pages
Song 1;1
Song 2;1-2
Song 3;2

If you import that into MobileSheets you get three songs. 
Song 1 and Song 2 are easy to handle: just crop them to show only that part of the page that is required.
Song 2: crop the pages that only the parts are shown that belong to Song 2
Then set "Page Scaling Mode = Fit Width" and "Display Mode = Vertical Scrolling" to get the two parts on the same screen.
Note that "Page Scaling Mode" and "Display Mode" have separate settings for portrait and landscape (both default and per song).