When you try to double-click on an Excel file from the desktop, you expect the file to open in it's own instance.
Repro: 1. Open a Windows Explorer window and navigate to a folder that contains Excel documents. 2. Double-click an Excel document to open it. Double click on a second Excel document to open it as well.
This is by design. By default, Excel will always open files in the same instance of Excel. Additionally, by default, if two instances are open it will always open in the original instance.
Using the Windows Start button, open two instances of Excel. Choose the instance in which you want the file to open and use File > Open to browse to and open the file.
If you are trying to display both files across monitors, you can use one instance of Excel and utilize the feature for “Arrange All” from the View ribbon. Then stretch the entire application over multiple monitors.