Excel - Opening Excel Files in Separate Windows
By default, Excel opens files in the same window. From time to time you might need to view two files side by side. This can be accomplished by manually opening an additional excel window using the process below.
Assuming you already have an Excel window open, press and hold Shift on your keyboard then Left click on the Excel icon on the taskbar at the bottom of your main display.
Click on Start > All Programs > Microsoft Office > Excel. (Excel always opens a new window when you launch it from the Start Menu.)
With a new window open, you can use File > Open or drag & drop your Excel file to quickly open it in the new window.
NOTE: There are disadvantages to using separate windows for files. Certain features will have limited functionality such as the copy/paste function and formula references.