Two Pages Side By Side in Preview.app
I have noticed that Preview displays the first page on its own in 2 Pages view mode.
Then the next two pages (if there are two pages in the document) are displayed side by side.
If you have a two page document, such as sheet music, that you want to display side by side then an easy fix is to insert a page at the beginning. I put a random one page document in front of the two I wanted to see together, but any way is fine.
iMac 27″, macOS 10.13