I've had this problem too. Here's what I did:
In Excel 2003, go to Tools -> Options
-> General tab.
Make sure the option, ‘Ignore other
applications’ is checked.
In Excel 2007 & 2010, Click the Office button
-> Excel Options -> Advanced.
Under General, check ‘Ignore other
applications that use Dynamic Data
Exchange’.
I don't know why this isn't built in.
Source: http://www.lytebyte.com/2008/05/13/how-to-open-two-excel-files-side-by-side-in-separate-monitors/
