I ran into this problem a couple of times. In other words, if I'm working on one Excel file, there are times when I would like to open another window. Of course you can view two different files in one Excel window using something like View -> Arrange All -> [the choose] Tiled, Horizontal, Vertical, or Cascade. But I'm talking about two separate windows side by side. And for some strange reason in Office 2010's version of Excel, this tiny little desire isn't as easy to figure out as one would think.
The easiest way to do this is to open two instances of Excel. In other words, open your first Excel file. Then go through the Start menu to open a second instance of Excel. Then open the Excel file you want to work with this new Excel window. That's it.
This solution came from here: http://answers.microsoft.com/en-us/office/forum/office_2010-excel/i-want-to-open-2-excel-files-in-separate-windows/f7588d9a-3c41-466f-8f01-3b6ab9661ae3 This forum also offers other options.