[itsme]

You can do everything you want in MobileSheets without the need to change the PDF or to create a new one:
Create a song in Mobilesheets with a.pdf. 
On the files tab of the song editor set page order to p. 73.
Press the "+" button, choose  "Local File" and select b.pdf.
Set the page order for b.pdf to 37 (or 37-... if required).
Crop the pages of the song to the sections that you want to see.
Additionally you can set page scaling mode to "Fit Width", display mode to "Vertical Scrolling" and "Remove gaps between pages" (also on the "display mode" window)
Keep in mind that "scaling mode" and "display mode" have independent settings for portrait and landscape orientation. 
Don't confuse the default settings for the complete library and individual settings per song for "scaling mode" and "display mode"