Click "File," "Open" and browse to your second Excel file. Double-click the file to open it. Repeat the steps for each new file you'd like to open in its own window.

These free add-ins for Word, Excel, and PowerPoint enable those apps to open and save their ODF counterparts (which typically have .ODT, .ODS, and .ODP extensions, respectively).

I also tested the plugin with a highly complex ODS file. In Excel, users load ODS files by clicking a special button in a toolbar rather than using the regular Open operation from the File menu.