I'm not quite certain how to answer this question. It's been possible to drag a window and have it snap to half of the screen with another on the other half in Windows for quite a while now. You could always do this manually before the automatic "snap to half screen" feature.
You can definitely have two Acrobat Reader windows open at the same time. You could also install an alternate PDF reader, and there are scads of them, and have one open in Acrobat Reader and the second open in Foxit Reader, PDF-XChange Editor, or similar.
There seems to be something missing from the original problem statement.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
~ Brian Vogel