Update: Excel 2013 and subsequent versions allows for split screen without using the below method. This is applicable to Office 2010.
Update 2: To use Excel 2003 in Split Screen; after you have the first file open by whatever method, hold Shift down and click on the Excel Icon on the Taskbar to open the second copy. Then use File -> Open to open the second file.
Want to use Excel like this;
Instead of like this;
Simply open a new Excel window from the Start menu, then open the required sheet from within the window (File -> Open). Just repeat for the desired number of windows you want open.
If you use Excel as much as I do on a daily basis, this is a big time saver.