I often work with dual monitors and recently needed to compare two Excel spreadsheets side by side. I found that although they showed as separate instances on the Windows Taskbar I couldn’t have them displayed on separate screens. After some investigation I found that to do what I wanted I had to force Excel to open each document as a separate instance of the application. To do this try the following:
In Explorer go to Tools > Folder Options.
Choose the File Types tab then find XLS under Extensions. Highlight it and click the Advanced button.
Select Open then the Edit button.
Now in the Application used to perform action field go to the end and add a space followed by "%1". Be sure to include the quotes. You also need to unselect Use DDE. OK all of your changes. Now when you open multiple Excel spreadsheets each will be a separate instance of the application so you can move them to separate monitors. The downside is that you’ll use more of your PC resources.
This issue has been addressed in Excel 2007 beta 2.