Here is what you do to open two seperate Windows in Excel:
To solve the problem you need to start two separate instances of Excel.
To do this:
Run Excel through the Start menu and open one instance -> Then click it again to open a second window. You can now use these two instances to open the files that you want to see.
If you double click the files when opening them, they will usually all open in the one instance of Excel!
Simpel as that 🙂