After that, make sure that the Open selected file types in Protected View option is selected. If so, tick all the Open checkboxes that are related to the old version of Excel. For example, you ...

Open Microsoft Excel. Go to “ File > Options > Trust Center.” Now, click on the Trust Center Settings button. Select Protected View from the left side. Uncheck all the options on the right ...

A file with the XLSX file extension is a Microsoft Excel Open XML Format Spreadsheet file. It's a ZIP-compressed, XML-based spreadsheet file created by Microsoft Excel version 2007 and later.

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.

Go to the File Types tab, scroll down to Microsoft Excel Worksheet (.xls not .csv) and click Advanced. Uncheck 'Browse in same window' (or 'Open Web documents' in place) and OK.