For some bizarre reason, after more than 20 years of existence, Microsoft Excel doesn't allow you to open two workbooks in separate windows. You would think that you could do that by going to "Tools/Options and then clicking the VIEW tab, and under SHOW, select "Windows In Taskbar". Even though this will open each new workbook in a new window,  instead of grouping them all into one, it still won't allow you to display those windows one next to the other, or on separate monitors. What could possibly make someone feel this was a good idea, is beyond me. It is a limitation which makes no sense. There are various tips on how to do this scattered across the web, but some of them don't really address the issue and others are way too complicated. However, the solution is simple.

Go to START Menu, open a copy of Excel. It will open on one of your monitors. It does not matter which one. Now, open a second copy of Excel the same way. (Do not doubleclick a file name to open Excel!) The second one will open directly on top of the first one, but there will be two Excel icons in the taskbar. Drag the top one to the other screen. You can now open two separate files on each of the windows.