I assume you have excel icon on desktop.

Start Excel from icon, and open 1st workbook;
Start again Excel from icon (Or from Start button when you have older than Windows 10. I'm not sure how excel behaves when to open it 2 times at program search in Windows 10) - the new instance of Excel is started - and open 2nd workbook.

Now you can resize both Excel instances indepently, place them on same screen not overlapping, drag one of them into another screen, when you have 2 of them, etc. The minus is, that sometimes you can't use PasteSpecial features between 2 instances.