I found something surprising - Excel 2010 doesn't easily support multiple windows. This makes it very troublesome to compare spreadsheets.
Good explanation at http://social.technet.microsoft.com/Forums/en/office2010/thread/749983b3-ba7e-49d3-99bf-f67d70245046 (If the link doesn't work, search for "Excel 2010 files won't open in separate window")
In short, you have to manually open 2 instances of Excel and open the files individually in them. Excel is still Multiple Document Interface (MDI) (opens all documents in single window) and only simulates new Single Document Interface (SDI) by showing open documents in the Task Bar. Word is already new SDI.