When I need to work in two separate Excel files on two separate monitors,
here's something I do:

StartRunExcel. Take that instance of Excel and drag it to one monitor
(downsize it first).
Again, click StartRunExcel; this will open a new instance of Excel.

Open your Excel files. You can now work in two separate files.

Remember: when working with dual monitors, you cannot copy/paste a formula
from one file to another.