If you launch multiple files by double-clicking their shortcut icons, or via the "Open" menu in Excel, the files will run simultaneously in one Excel window.

How to Open an XLS File in Excel. XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save ...

How to install to avoid problems, making documents open when double-clicked and resolving keychain issues.