Open both files. Go to the View tab on your menu. Select Arrange All and then Horizontal or Vertical depending upon how you want to work on the files. Select one and if the appearance isn't useful, open up Arrange All and select the others. You can try Tiled or Cascade if you like but I prefer Vertical for a side by side comparison. You will now have both files open side-by-side and can compare or copy/paste or use formulas between the two files as needed.
Also, you can select View Side by Side from the View menu in the ribbon. If you want both worksheets to scroll at the same time, select Synchronous Scrolling just below that (see the second tip below).
Greg G's answer will allow you to see two or more files, but you'll have to do some rearranging of the windows to see and work on both. Gospiele's tip is only for separating the different files into individual buttons on the taskbar, which can also be helpful but not what you asked. The above steps will give you two or more workable and viewable windows within Excel.
I don't know if you can set this mode as a default mode. I didn't see anything in the Options/Preferences menu to be able to set this permanently, but you might not need to use it all the time. It only takes two or three clicks to do, anyway.
For more details, you can check out the following:
http://www.dummies.com/how-to/content/how-to-arrange-windows-in-excel-2010-workbooks.html
http://www.addictivetips.com/microsoft-office/word-2010-synchronous-scrolling-view-side-by-side/