Once you have the two instances of excel running as stpbby suggested you can either use the aero snap feature if available and grab them each by it's respective title bar and bang them against the left and right side of your screen, or alternately right click on the taskbar and choose show windows side by side.
I don't know what version you are using but in Office 2010 it's very simple. Open one spreadsheet and then go to File\Open and open a second one. Then go to the VIEW panel and click "View Side by Side". Done.